If your ants are very small black ones you're probably dealing with Monomorium Minimum or Tapinoma Sessile.
They are in your bathroom for the water, or they have picked up a scent that smells like food. Many products in your bathroom could be attracting the
ants, especially if you have females in the house that apply make-up products in the bathroom.
A tube of silicone chalking only costs a couple bucks, and going around all the edges, corners, nooks, and crannies, will take less than an hour.
Good quality chalking is about your best defense for the interior.
Keeping food sources away from the exterior wall will help a lot, but keep in mind that ants will travel quite a ways. So put some ant killer down
around the area outside the bathroom to make sure they're not coming in from there.
And a tube of exterior chalking applied to any tiny cracks or holes outside should solve your problem.
If your dealing with ants that have set up a colony in your walls you might be in for a bigger problem, but baits and traps can handle those.
Beer_Guy is right on.
Boric Acid is fantastic, and you can get it at any pharmacy for about $2.00.
You may also want to try non-toxic baits so you can find out where they live.
Place a few drops of honey near an area with ants and see if they form a trail. If nothing happens, try a bit of bacon fat.
Track the little suckers back to their nest, and kill without mercy.
Boiling water can be very effective when poured on an outdoor nest, and you don't need to mix anything up.
[edit on 12/3/2007 by anxietydisorder]