Here are some other things I thought of as soon as I hit the "post" button for my last answer--make sure each cat has his own litter box. Some cats don't like to use the same box as other cats. It's like going into a public restroom where the previous person hasn't flushed. Don't use covered boxes--some cats don't like those. Make sure the box is in a private, quiet area and that the cats don't harrass each other while one is using the box. Cats feel most vulnerable when using the box. If this is becoming a very stressful problem as you've said, try closing your cats in a room with food/water and a box. Check on them twice a day. This will retriain them to use the box since there's nowhere else to "go". Of course don't use a room with furniture or carpeting--a bathroom is ideal. Take out towels and rugs. Good luck and please check with your vet to rule out health concerns.