If you go up into the attic during the day, you should be able to see tiny openings in which bats can enter a home. Let your eyes get used to the dark first...that way they will be more sensitive to the light. Once you've spotted the locations, spray paint them so that you can find them later. Heavy wire screening works to keep them out, or hardware cloth.

I've not heard of any electronic devices that really worked.

The people that sell the sonic repellant devices will tell you they work, but I've not seen anything from people that actually study bats, mice, squirrels, etc. that will support that. As Rhizo suggests find out how the bats are getting into the attic and seal those entry points, when the bats are not in the attic.

Mark, for those tiny spaces that can be common in older homes - those you think would be too small to be an entry point but are not :) - you can pick up a package of steel wool at your hardware store, pack it into place with a screwdriver. Or use caulk - a bat is less likely to chew through caulk like a mouse would.

I understand why you wouldn't want them in your attic, but a population in the neighborhood is a good thing, they keep down the flying insect numbers and that includes mosquitoes.

Thanks for all the updates. I understand about trying to find the small crevices in the attic but where they are is impossible for me to get to from the attic. The roof line is so small that I cannot get there. I was hoping that there was a device that emitted a sound that would "drive" the bats out. I may try to climb up in the spring and see what I can from inside and also look outside.

Thanks,

Mark