Found a Mouse in Your House? Here's Exactly What to Do

You were in the kitchen at 11 PM and something ran under the refrigerator. Your heart jumped. Now what? Here's a calm, step-by-step guide for what to do in the first 24 hours after discovering a mouse — and why acting quickly matters more than you might think.

First: Don't Panic — But Don't Wait Either

Seeing one mouse doesn't necessarily mean you have dozens. It might be an early-stage problem that's fast and inexpensive to resolve. But here's the critical fact: mice breed rapidly. A single female mouse can produce 5-10 litters of 6-8 pups per year. A pair of mice that enters your Philadelphia home in October can realistically become 20-30 mice by February. Early action is the difference between a minor inconvenience and a significant infestation.

Step 1: Identify Signs of How Long They've Been There

Before doing anything else, do a quick inspection. Look for:

• Droppings: Mouse droppings are dark, rod-shaped, about the size of a grain of rice. Finding them in multiple locations suggests an established population; finding only a few in one location suggests a recent entry.
• Gnaw marks: Check food packaging in your pantry. Gnawed cardboard or bags is a strong sign of more than one visit.
• Nesting material: Check behind the refrigerator, under the stove, and in cabinet corners. Shredded paper, fabric, or insulation is a definitive sign of an established nest.
• Grease marks: Dark smear marks along baseboards at floor level indicate regular mouse travel routes.

Step 2: Immediately Secure Your Food

Move everything in vulnerable packaging — flour, sugar, cereal, rice, pasta, pet food — into sealed hard-sided containers or into your refrigerator. Do this before going to bed. Mice forage primarily at night and will return to any food source they found on the first visit.

Step 3: Identify Likely Entry Points

Mice don't appear from nowhere — they entered through a gap somewhere. The most common entry points in Philadelphia homes:

• Where utility pipes (gas, water, conduit) enter through the foundation or walls
• Gaps behind kitchen cabinets where pipes come through the wall
• Cracks in the mortar of brick or stone foundations
• Under the door at the base (mice can squeeze under a door with a gap larger than a pencil)
• Around where the sill plate meets the foundation
• Openings in the roofline or soffit (for attic-dwelling mice)

Look with a flashlight. You're looking for gaps larger than 6mm (roughly the diameter of a pencil) — anything that size can admit a mouse.

Step 4: Set Snap Traps in the Right Places

Snap traps are the fastest-acting and most effective trapping method for a fresh mouse problem. Place them:

• Perpendicular to walls, with the trigger end touching the baseboard (mice run along walls, not through open spaces)
• Behind appliances (refrigerator, stove, dishwasher)
• Under sinks
• In pantry corners near signs of activity

Use peanut butter as bait — it works better than cheese because mice can't remove it from the trigger without setting it off. Place at least 3-4 traps. One trap catches one mouse. Most people underestimate how many traps they need.

Step 5: Decide Whether to Call a Professional

Call a professional if:

• You catch 2 or more mice within 3 days (this indicates a larger population)
• You find droppings in multiple rooms or areas
• You're hearing noises in the walls regularly
• Traps aren't catching anything but you're still seeing evidence (mice that avoid traps have often encountered them before)
• You have children, immunocompromised family members, or asthma in the household (mouse droppings and urine are significant health hazards)

What to Do About the Health Risk

Mouse droppings, urine, and nesting material can transmit hantavirus, salmonella, and other pathogens. Don't vacuum or sweep dry droppings — this aerosolizes particles. Instead: wear gloves, dampen droppings with a paper towel soaked in diluted bleach (1 part bleach to 9 parts water), pick up with the paper towel, and dispose in a sealed bag. Wash hands thoroughly after.