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How to identify it
- Large and robust, 7 to 18 ounces, with a blunt muzzle and small ears that don't reach the eyes when folded forward.[1]
- The tail is shorter than the head and body (reaching only to the middle of the skull when pulled forward), dark above and pale below.[1]
- Burrows along foundations and under rubbish and woodpiles, with an entrance and escape holes.[1][4]
- Active burrow holes are 2 to 3 inches wide, kept clear of debris, often with rat hairs around the entrance, according to the Internet Center for Wildlife Damage Management (ICWDM).[4]
- Droppings can be as large as 3/4 inch long and 1/4 inch thick with blunt ends; a rat leaves about 40 to 50 a day, usually clumped in small groups.[4]
- Greasy smudge marks on beams, pipes and walls show regular travel routes, left by oil and dirt rubbing off the fur.[4][1]
Look-alikes
Where it lives
Throughout the 48 contiguous states, mostly at lower elevations; in buildings they usually live in basements and ground floors.[1]
Biology and life cycle
Litter numbers differ by source. UC IPM says a female has 4 to 6 litters a year and can wean 20 or more young; ICWDM gives 3 to 6 litters a year of 6 to 12 young each, born 21 to 23 days after conception.[1][5]
Young rats are independent at about 3 to 4 weeks and can breed at 3 months. Breeding often peaks in spring and fall.[5]
Foraging range: UC IPM says Norway rats eat at 100 to 500 feet from the nest, while ICWDM says a rat covers an area about 100 to 150 feet across each day and seldom goes more than 400 feet from its burrow. Both agree they prefer cereal grains, meat, fish and nuts but eat almost anything.[1][5]
They are active mostly from dusk on. Eyesight is poor, so they find their way by smell, taste, touch and hearing, and they tend to live on the lower floors of buildings even though they climb well.[5]
Rats are wary of new objects, so they may avoid traps and baits for days or weeks.[1][5]
Damage and signs
- Burrowing can undermine foundations and slabs; they gnaw wood, plastic and even soft metals such as copper and lead.[1]
- They can enter through sewers and broken drains and squeeze under a 1/2-inch door gap.[1][6]
- They are strong swimmers that ICWDM says can stay afloat for up to 72 hours, and they move into sewer systems at outlets and through manholes, catch basins, broken pipes and drains.[6]
Health risk
CDC says rats and mice carry many diseases that reach people through their droppings, urine and saliva (breathed in as contaminated air or eaten in contaminated food), through bites and scratches, and through ticks, mites and fleas that have fed on them. Many infected rodents look healthy.[7]
Control
Keep them out
Exclusion is the most successful and permanent rodent control: seal every gap larger than 1/4 inch with metal or concrete, including around pipes, vents and utility lines, and make doors, windows and screens fit tightly. Foam, plastic, rubber and wood are gnawed through.[2][1]
Sources set different hole sizes. ICWDM says rats are kept out by sealing every opening larger than 1/2 inch across with concrete mortar, 24-gauge galvanized sheet metal or 18-gauge hardware cloth; UC IPM seals anything over 1/4 inch. CDC's seal-up list adds gaps around water heater and furnace pipes, floor drains, and the roofline at rafters, gables, eaves and soffits.[6][1][8]
Sanitation
Remove food sources such as garbage, pet food and fallen fruit, and clear woodpiles and debris that hide burrows.[1]
CDC advises keeping woodpiles 100 feet or more from the house and at least one foot off the ground, keeping compost bins at least 100 feet away, and not leaving pet food or water bowls out overnight.[8]
Traps and physical control
Set rat-sized snap traps along walls and runways between shelter and food, baited with a pea-sized amount of nuts, dried fruit or pet food; expect rats to avoid new traps for a while.[1]
Pre-bait first. ICWDM says to leave traps unset until the bait has been taken at least once, and CDC also advises baiting traps without setting them so rats get used to them. In research ICWDM cites, expanded-trigger snap traps caught almost twice as many rats as narrow-trigger ones.[6][9]
CDC's rule for knowing when you are done: if nothing is caught for a week and no new droppings, urine spots or gnawing appear, the rats are gone. CDC advises against glue traps and live traps because frightened rodents urinate, which raises your risk of getting sick.[9]
Pesticides
Rodenticide baits sold to the public are limited to wax block, gel or paste baits packaged in or with ready-to-use, disposable bait stations; poisoning inside buildings can leave dead rodents in walls, which is one reason trapping is preferred indoors.[1][2]
CDC recommends snap traps for rats around the home and poison or bait stations only when an infestation persists, using EPA-registered products kept out of reach of children and pets.[9]
Diseases linked to rats
Leptospirosis: CDC lists rodents, including rats and mice, among the animals whose urine spreads the Leptospira bacteria, which can survive in water or soil for weeks to months. Without treatment it can lead to kidney damage, meningitis, liver failure and trouble breathing.[11]
Rat-bite fever: CDC says it spreads through bites, scratches or contact with rodent saliva or urine, and also through contaminated food or water. It is rare in the United States, but symptoms usually start 3 to 10 days after contact and need prompt antibiotic treatment.[12]
Hantaviruses: CDC says people catch them from rats and mice, mainly through urine, droppings and saliva. Seoul virus, a hantavirus that causes hemorrhagic fever with renal syndrome, is found worldwide, including in the United States.[13]
After rats have been in a space, CDC says not to sweep or vacuum droppings or nests, since that puts virus-laden droplets in the air. Wear gloves, soak the mess with a disinfectant or a fresh 1-to-9 bleach solution for 5 minutes, and wipe it up with paper towels.[10]
Common questions
- Norway rat or roof rat?
- Pull the tail forward over the back: a Norway rat's reaches the middle of the skull, a roof rat's reaches the snout.[1]
- How far do Norway rats travel from their nest?
- Not far, but sources give different figures: UC IPM says they forage 100 to 500 feet from the nest; ICWDM says they seldom go more than 400 feet from the burrow.[1][5]
- How do I know the rats are gone?
- CDC says keep trapping until nothing is caught for a week and you see no new droppings, urine spots or gnaw marks.[9]
- What size hole can a rat get through?
- UC IPM says rats can squeeze under a door with only a 1/2-inch gap. ICWDM's exclusion advice is to seal openings larger than 1/2 inch, and UC IPM's is to seal anything over 1/4 inch, which also stops mice.[1][6]
Sources
- University of California Statewide IPM Program, “Pest Notes: Rats, Publication 74106 (revised June 2024).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnrats.pdf (accessed 2026-11-11)
- University of California Statewide IPM Program, “Pest Notes: House Mouse, Publication 7483 (revised July 2025).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnhousemouse.pdf (accessed 2026-11-11)
- iNaturalist, “Rattus norvegicus taxon page (order and family).” www.inaturalist.org/taxa/44576 (accessed 2026-11-11)
- Internet Center for Wildlife Damage Management, “Norway Rats: Identification.” icwdm.org/species/rodents/norway-rats/ (accessed 2026-11-11)
- Internet Center for Wildlife Damage Management, “Norway Rat Biology.” icwdm.org/species/rodents/norway-rats/norway-rat-biology/ (accessed 2026-11-11)
- Internet Center for Wildlife Damage Management, “Norway Rat Damage Prevention and Control Methods.” icwdm.org/species/rodents/norway-rats/norway-rat-damage-prevention-and-control-methods/ (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “Controlling Wild Rodent Infestations.” www.cdc.gov/healthy-pets/rodent-control/index.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “How to Seal Up to Prevent Rodents.” www.cdc.gov/healthy-pets/rodent-control/seal-up.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “How to Trap Up to Remove Rodents.” www.cdc.gov/healthy-pets/rodent-control/trap-up.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “How to Clean Up After Rodents.” www.cdc.gov/healthy-pets/rodent-control/clean-up.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “About Leptospirosis.” www.cdc.gov/leptospirosis/about/index.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “About Rat Bite Fever (RBF).” www.cdc.gov/rat-bite-fever/about/index.html (accessed 2026-11-11)
- U.S. Centers for Disease Control and Prevention, “About Hantavirus.” www.cdc.gov/hantavirus/about/index.html (accessed 2026-11-11)