Signs of a Rodent Infestation: How to Tell If You Have Mice or Rats

The most common signs of rodent infestation are droppings, gnaw marks, greasy rub marks along walls, nests of shredded paper or fabric, and scratching in the walls or ceiling at night. Droppings are the easiest to spot. Mouse droppings are 1/8 to 1/4 inch long and shaped like a rod, while rat droppings are 1/2 to 3/4 inch long and shaped like a capsule, according to Montgomery County’s guide to rats and mice.

One sign can mean a mouse passing through. Several signs in the same place mean they’re living there, and mice breed fast. A female house mouse can have 5 to 10 litters a year of about 5 or 6 pups each, according to UC IPM’s 2025 guide to house mice. The sooner you find how they get in, the smaller the job.

Signs of mice and rats at a glance

Here’s how the common signs differ. The details come from the EPA, NYC Health, Montgomery County and the University of California’s guides to mice and rats.

Droppings tell you the most

Where you find droppings tells you where the nest is. House mice seldom go more than 30 feet from their nest and their food, according to UC IPM. So droppings in the drawer next to the stove usually mean a nest within a few feet: behind the stove, under the cabinet or inside the wall. Norway rats travel much farther, about 100 to 500 feet from the nest, so a rat in the basement may live in a burrow in the yard or under the sidewalk.

To tell old droppings from new ones, clean up the ones you find, then check the same spots in a day or two. New droppings mean rodents are still active. Don’t sweep or vacuum them. The CDC says that can put germs into the air, and our guide on how to get rid of rodents walks through the safe way to clean up.

Gnaw marks, rub marks and runways

Rodents chew to get to food and to get in. Mice leave small holes in cereal boxes, rice bags and pet food. Rats chew bigger things: wood, wires and the lids of garbage cans. Fresh shavings under a chewed spot mean the damage is new.

Rats also use the same paths every night. Where their fur brushes walls and pipes, it leaves dark, greasy rub marks. Outside, NYC Health points to worn paths in grass and holes in the ground, in sidewalks or along building foundations.

Nests, noises and smell

Mice build nests from finely shredded paper or fabric in sheltered spots, close to food: inside walls, under kitchen cabinets and in ceilings. Look for a loose ball of paper bits, cotton or insulation.

Rodents are most active at night, so the first sign is often a sound: scratching, running or gnawing in the walls or ceiling after dark. A musky smell is common in large or long-term mouse infestations, according to UC IPM, so a strong smell usually means they’ve been there a while.

Mice or rats? How to tell

• Droppings: up to 1/4 inch and rod shaped means mice. 1/2 to 3/4 inch with blunt ends means rats.
• Where: mice live inside, in walls, cabinets and ceilings, close to food. Norway rats live in basements, sewers, trash areas and courtyards, and burrow under sidewalks and foundations.
• The animal itself: an adult house mouse is 5 to 7 inches long, including a 3 to 4 inch tail, and weighs about half an ounce. A Norway rat averages about 16 inches with its tail and weighs half a pound to just over a pound. Our guide to Norway rats shows how to tell them apart.
• The gap they use: mice can squeeze under a gap 1/4 inch tall. Young rats can get under a door with a 1/2 inch gap, according to UC IPM.

One sign or a real infestation?

Here’s a simple example. Say you find a few small, rod-shaped droppings in the drawer next to the stove and a bag of rice with a chewed corner in the cabinet below. That’s mice, and the nest is probably close, behind the stove or under the cabinet. The gas line or water pipe going through the wall is the most likely way in, and it’s the first thing to seal.

Our advice: treat fresh droppings in the kitchen as a reason to act now, not a sign to watch. Signs can’t tell you how many rodents there are or where every gap is. A few droppings under the sink can come from one mouse, or from a nest in the wall that serves three apartments.

What to do next

If you rent, tell your landlord or super in writing. In New York City, owners of buildings with three or more apartments must inspect every unit for mice, rats and roaches each year and fix problems using Integrated Pest Management, under Local Law 55. If the owner doesn’t act, file a complaint with 311.

If you own the building, NYC Health says owners must keep their properties free of rats. Learn what attracts mice and how to get rid of rodents for the steps you can take now.

NYC Extermination starts every rodent job with a free on-site inspection. We look for droppings, gnaw marks, runways, nests and every gap they use, inside and outside. Then we remove them, seal the entry points and check back. See our rodent control service and pest control for apartment buildings, or call (347) 988-2315.

Frequently asked questions

What does a rodent infestation smell like?

A stale, musky smell, strongest near nests and in closed spaces like cabinets and wall voids. Find the source: a nest behind a cabinet or a mouse that died inside a wall can both cause it. Wear gloves and follow the CDC’s cleanup steps when you remove it.

How do you get rid of a rodent infestation?

Seal the gaps they use, take away food and hiding spots, then trap the ones inside. Trapping without sealing rarely ends it, because new rodents keep coming in. Our guide on how to get rid of rodents covers each step.

How long do rat infestations last?

As long as rats have food, shelter and a way in. A female Norway rat can have 4 to 6 litters a year, according to UC IPM, so a problem left alone grows. Seal the entry points, remove what feeds them and keep checking for new droppings until you find none.

Read Customer Reviews