House Mouse: Identification & Signs of Mice

The house mouse (Mus musculus) is the mouse most often found inside homes and businesses in Illinois. It isn't a native species: it arrived from Europe and lives almost entirely alongside people.

• Size: a body about 2½–3½ inches long, plus a tail of about the same length. Adults weigh only ½–1 oz.
• Color: dusty gray to light brown on top, with a slightly lighter gray or buff belly (not bright white).
• Features: a pointed snout, large rounded ears, small black eyes and a long, nearly hairless, scaly tail.
• Abilities: house mice are excellent climbers, can jump about a foot straight up and run along wires and pipes.

House mice are mostly active at night and nest close to food, using shredded paper, insulation and fabric. They eat grains and seeds but will try almost anything in a kitchen or pantry. A female can have five to ten litters a year, each with around five or six young, and the young can breed at about two months old. That's why a couple of mice in October can become a real infestation by winter.

A mouse can squeeze through a gap about ¼ inch wide, roughly the size of a dime, and it will gnaw a small gap until it fits. Around Chicago, mice move indoors in the fall when temperatures drop, usually from mid-September through November. Common entry points include:

• gaps where pipes, cables and A/C lines enter the house
• worn garage door seals and missing door sweeps
• foundation cracks, weep holes and gaps around basement windows
• openings around dryer vents and under siding

Once inside, they head for quiet, warm spaces: attics, basements, wall voids, behind appliances and inside kitchen cabinets.

Signs of Mice in Your House

Seeing a live or dead mouse is the obvious sign, but most people notice something else first:

• Droppings: small, dark and pointed at the ends, about ⅛–¼ inch long, often along walls, in drawers or under the sink
• Noises: scratching or scurrying in walls and ceilings, especially at night
• Gnaw marks on food packaging, baseboards, wiring or plastic
• Nests of shredded paper, fabric or insulation in hidden corners
• A musty smell in cupboards or closed spaces
• Grease marks along the routes they run every night

Mouse or Rat?

A young rat can look like a large mouse, so check the details:

Learn more on our Norway rat page.

Illinois also has two native mice that come indoors in late fall: the deer mouse and the white-footed mouse. They're common in homes near forest preserves, parks and open fields. They're easy to tell apart from house mice:

• a sharp two-tone coat: brown or reddish-brown on top and bright white underneath
• white feet
• large, bulging black eyes
• a furry tail that's dark on top and white below

The difference matters for your health. Deer mice and white-footed mice are the main carriers of hantavirus in the US, and white-footed mice are also hosts in the life cycle of the ticks that spread Lyme disease. If you think you have deer mice, take extra care when cleaning up (see below).

• Seal gaps larger than ¼ inch with steel wool or copper mesh, then caulk over it. Expanding foam alone won't stop mice, because they chew straight through it.
• Fit door sweeps and replace worn garage door bottom seals.
• Store food in glass, metal or hard plastic containers, including pet food and birdseed.
• Keep firewood off the ground and well away from the house.
• Trim shrubs and clear clutter from around the foundation.
• Check in late summer, before the fall rush, when it's easier to find and seal entry points.