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The answer is possible, not predictable
Rats can kill mice, but a homeowner should not treat that as an expected service provided by a rat infestation. Laboratory research has documented wide differences in mouse-killing behavior among strains and sexes of rats. In one study of inbred laboratory rats, the frequency varied dramatically across strains. That finding supports a careful conclusion: mouse killing can occur, but it is not a reliable forecast of what any particular wild rat will do in a garage, pantry, attic, or yard.
Real homes add food sources, shelter, escape routes, multiple animals, and species that a laboratory setting cannot reproduce. A large rat may compete with or displace mice without leaving clear evidence that it killed them. A mouse population can also persist in another part of the structure while rats use a separate route. Do not infer that a mouse problem has solved itself because a rat appeared, or that a rat must be present because mice have disappeared from one room.
Competition is not a control plan
Rats and mice may compete for food, nesting material, and sheltered travel routes, but competition does not make either species desirable indoors. Both can contaminate food, gnaw materials, and exploit building gaps. A home that supports one kind of rodent often has accessible food, water, shelter, or entry points that can support another. CDC recommends inspecting the interior and exterior for openings and conditions that allow rodents to enter or remain.
Trying to use rats, cats, scent products, or other animals to control mice creates a new problem instead of repairing the original one. It can leave dead animals in inaccessible areas, expose pets to injury, and hide the evidence needed for a proper inspection. The sensible response is species-neutral at first: document signs, secure food, remove shelter, close access, and use an appropriate monitored removal method when current activity is confirmed.
Tell signs apart before selecting equipment
Rat and mouse evidence can look similar from a distance, but equipment choice depends on knowing what you are dealing with. UC IPM notes that mice and small juvenile rats may not activate a rat snap trap. Examine the size and location of droppings, the scale of gnawing, travel routes, and the kind of entry opening, but do not identify an animal from one ambiguous mark. Photographs and a professional inspection can help when the distinction affects a removal plan.
Look for patterns rather than one clue. Mice often use small protected interior spaces, while Norway rats may be associated with ground-level burrows and roof rats with elevated routes, but no single sign proves the species. The same home can host more than one rodent. If a trap catches an animal, that is useful confirmation for the area where it was set, not proof that every sound or dropping elsewhere has the same source.
Repair the conditions both species use
The most durable response works regardless of whether you found a mouse, a rat, or both. Store food and pet feed in closed containers, clean crumbs and spills, keep garbage covered, correct leaks, and reduce clutter that creates nesting cover. Outside, make foundation edges, vents, door bottoms, and utility penetrations visible and maintainable. CDC recommends sealing gaps and holes around common interior and exterior entry points.
A good exclusion repair is specific to the feature. Use durable material for a damaged screen, service-line gap, or foundation opening, and preserve needed drainage, ventilation, and utility functions. Do not cover an active vent or duct with makeshift material. If the access is near a roof, electrical line, plumbing, or structural defect, bring in the appropriate professional. Rat-versus-mouse questions become less important once the building is no longer an easy route for either.
Use removal methods that fit the evidence
If fresh activity continues after sanitation and initial exclusion, use a controlled method matched to the confirmed rodent and location. CDC recommends snap traps around homes, states that traps should be out of reach of children and pets, and advises checking them regularly. A rat-sized trap is different from a mouse trap; do not substitute based on appearance alone. Place equipment where signs indicate travel, not in the open center of a room.
Avoid using poison or a predator as a shortcut for uncertainty. Any rodenticide must be EPA-registered and used exactly according to its label, with careful protection for people, pets, and wildlife. When activity is in a wall, attic, duct, or inaccessible location, a qualified professional can identify the route and manage the work. The goal is not to decide whether rats will kill mice. It is to remove the conditions that let either rodent occupy the home.
Keep the evidence separated by location
Do not merge every rodent sign into one story. A mouse caught in a kitchen trap and a gnawed attic vent may represent different species, routes, or times. Label photographs and notes by room or exterior feature, then compare fresh evidence after each repair. This prevents the common mistake of escalating the treatment everywhere when only one access point is active.
If a professional visits, show the location-specific record instead of leading with a theory about rat behavior. It allows the inspection to begin with observable facts and makes it easier to choose the correct trap, exclusion material, and cleanup scope. Whatever interaction rats and mice may have, the homeowner’s most reliable control is an organized response to the evidence actually present.