For those of you who have battled the problem for years with little permanent success, there is hope! A good trust worthy exterminator can cure the problem. A good exterminator will start with meticulously filling ALL the possible entry points around the perimeter of the home. This will generally take several hours and possibly several trips. If your exterminator spends 30-40 minutes setting out some poison and fills one or two holes around the home, they probably are saving some work and revenue for a few months from now when you call back with the same problem. A good job takes time and generally a great deal of filler. Mice can fit through a gap of less than 1/4" or through a hole less than 3/8" diameter. There are many openings around the exterior of a home that are large enough to let mice and other critters in.
When seeking out an exterminator, find one who will do a thorough job, NOT the cheapest one. Hiring the cheapest one can cost you the most money and aggravation because you will have to call them back over and over again and never completely solve the problem.
Ask the exterminator:
1) To describe the services they provide for the quoted fee.
2) Ask them how much time they generally spend on a job
3) Finally, ask them about their guarantee. (Anything less than a year is inadequate.)
Note: It has been known that some unscrupulous exterminators will fill a few hole but intentionally leave opening to ensure repeat business.
Although this is not mice it is an interesting photo of a recent find. A wasp nest that was built above the suspended ceiling in the floor joist of a home.