Thanks for being an expert.

I am a new owner of a semi-detached house. I have heard very occasional skittering in my living-room ceiling and suspect mice. I have looked both on the outside and inside the house, but can find no obvious entry point. Could the mice be in the walls and/or coming in from the neighbouring house? Suggestions? I have a cat whom, I'm sure, would kill the mice if she could get at them. Also, the location of the skittering coincides with the major beam on the living-room ceiling. How freaked should I be about that?

Thanks, again, for all your advice.

I have never heard of mice doing structural damage, so you can quit worrying about the beam. You still want rid of the filthy things. You could kill off the current population with traps or poison, but unless you track down where they came in, more will come back, at least next fall when they are looking for warm winter quarters.

Plumbers and electricians are generous about the size of holes they make for pipes and wires. Even if there isn't room for a mouse to squeeze through, they can gnaw the hole bigger. I finally rid my house of mice by cutting little sheets of aluminum to just fit over the pipes and wires and gluing them to the drywall where the pipes came through. It they get into your crawl space or basement, they can follow wires or pipes up to the attic. The first step is to check the foundation, and cement shut any holes you find. Make sure the screens are in good shape on any vents. Go over the whole house. If they have not been using a small hole, caulking it may be good enough to stop them from finding it and using it. Every hole you plug will at least stop cold air from coming in.

There are ultra sonic pest repellents that might work too, but your cat might not like them. Some people have good luck scattering moth balls in the attic.