Rodents
Is It a Mouse or a Rat?
By the Clark Pest Entomology Team
Last updated: October 28, 2026
Quick answer: Size tells mice from rats. A mouse is about 5 to 7 inches with its tail. Rats are larger. Roof rats favor attics and wall voids, while Norway rats stay low in basements and burrows. Match what you found below, then open that guide.
Which Rodent Do You Have?
Clark covers three common rodents in homes and businesses across California and Northwestern Nevada. Tap the one that sounds closest.
House Mouse Light brown to gray. It is about 5 to 7 inches long with the tail and can squeeze through a gap just over 1/4 inch.
Norway Rat Large and stocky with a blunt muzzle. It digs burrows along foundations and under rubbish or woodpiles. Indoors it usually stays in basements and on ground floors.
Roof Rat Smaller and sleeker than a Norway rat, with a pointed muzzle and large ears. It nests above ground in trees and dense shrubs, and in attics and wall voids indoors.
Quick Field Tests
Check these clues before you buy a trap. The right trap and the right spot depend on the animal. Our rats guide covers both rat species side by side.
| Clue | House mouse | Norway rat | Roof rat |
|---|
| Size | About 5 to 7 inches with the tail | Larger and stockier, 7 to 18 ounces | Smaller and sleeker, 5 to 10 ounces |
|---|
| Muzzle and ears | Go by size first | Blunt muzzle. Folded ears do not reach the eyes | Pointed muzzle. Folded ears reach the eyes |
|---|
| Where it nests | Sheltered spots, in nests of shredded paper or similar fibers | Burrows along foundations. Basements and ground floors indoors | Shrubs, trees and dense plants outside. Attics, wall voids and ceilings indoors |
|---|
| Gnaw marks | About 1/12 inch wide | About 1/6 inch wide | About 1/6 inch wide |
|---|
| Where to set traps | Along walls, behind objects and in dark corners | Close to walls, behind objects and in dark corners | Off the ground, on ledges, shelves, branches or fences |
|---|
Signs You Have Rodents
Droppings, gnaw marks and rub marks
Mice leave droppings, fresh gnaw marks and tracks. Mouse droppings are small and rod-shaped with pointed ends. Rats leave rub marks along walls. Rat droppings may turn up around pet dishes and pet food storage.
Noises, nests and odors
Mice are mostly active at night and have a musky odor. Their nests are made of finely shredded paper or other soft fibers.
Paths along walls
Mice travel along walls and objects on the floor. Those paths stay free of dust and cobwebs.
Are Rodents a Health Risk?
Yes. Rodents can spread disease, so the details matter.
Diseases rodents can spread
According to UC IPM, rats can spread typhus, leptospirosis, rat bite fever and foodborne illness such as salmonella. House mice can spread salmonella too. Plague is rare in house rats and mice but does occur in wild California rodents. The CDC says plague occurs naturally in parts of the western United States, and sick squirrels or prairie dogs can signal it. Rats and mice that live around people do not carry rabies.
In California, hantavirus comes from deer mice
The California Department of Public Health names the deer mouse as the source of Sin Nombre hantavirus in California. UC IPM says house mice have not been found to carry it. The CDC names the deer mouse, white-footed mouse, rice rat and cotton rat as the North American rodents that spread the hantaviruses behind hantavirus pulmonary syndrome. House mice and the two rats in this guide are not on that list.
Clean up droppings the safe way
Never sweep or vacuum droppings. That can release the virus into the air. The California Department of Public Health says to air out the space for 30 minutes first and wear gloves. Then soak the droppings with disinfectant or a fresh 10 percent bleach solution for at least 5 minutes and wipe them up with paper towels.
What to Do First
Start by sealing the building. The Nevada Department of Agriculture says mice can enter any opening larger than 1/4 inch and rats any opening larger than 1/2 inch. UC IPM says to seal every hole larger than 1/4 inch to keep out both. Use sheet metal or concrete for a lasting seal. Stainless steel scouring pads work as a short-term plug. Rats can chew through plastic, foam, wood and caulk, so those make poor plugs.
Cut off their food and water and take away places to hide. Clark's mouse guide says good sanitation is needed for long-term control.
For traps, UC IPM calls snap traps the safest and most effective way to catch rats. Use a pea-sized bit of bait. Peanut butter is the most popular bait for mice. Keep traps away from children and pets. See the UC IPM house mouse guide and the UC IPM rat guide for trap spacing and placement.
Be careful with poison. A poisoned rodent can die inside a wall and cause a bad odor. UC IPM also warns that a pet that eats a poisoned rodent can be poisoned too.
Clark's Expert Insight
Find the entry holes before you set a single trap. Traps only catch the rodents already inside. Sealing keeps the next ones out. Look along the foundation, around pipes and vents and where utility lines meet the wall.
Rodent Questions Homeowners Ask
How can I tell a Norway rat from a roof rat?
Check the face and the nesting spot. A Norway rat has a blunt muzzle and a stocky body. A roof rat has a pointed muzzle, large ears and a sleeker build. Roof rats nest high in attics and trees. Norway rats nest low in burrows and basements.
How small a hole can a mouse get through?
A mouse can squeeze through an opening just over 1/4 inch wide. UC IPM advises sealing every crack and opening larger than 1/4 inch. Young rats can also slip under a door with a 1/2-inch gap.
What do mouse droppings look like?
Mouse droppings are small, smooth and rod-shaped with pointed ends. Clark's mouse guide puts them at about 1/8 to 1/4 inch long. Wet them down and wipe them up. Do not sweep or vacuum.
Do house mice carry hantavirus?
UC IPM says house mice have not been found to carry hantavirus. The California Department of Public Health names the deer mouse as the source in California. Treat any droppings with care until you know what you are dealing with.
Should I use traps or poison?
UC IPM names snap traps as the safest and most effective way to catch rats. Poison can send rodents into walls to die and is toxic to pets and wildlife. Set mouse and Norway rat traps along walls. Set roof rat traps off the ground.
When should I call a professional?
Call when traps keep catching rodents, when you hear them in the walls or attic or when you cannot find where they are getting in. A technician can find the entry points and build a plan for your home or business.