I'm so sorry you have to go through this.

Have you ever read the "Unabridged Guide to Litterbox Problems" from Cats International? I keep copies at the shelter and hand them out to people just like you who love their cats but still need to take action.

If you have not, read it before you go to bed tonight for ideas

http://www.catsinternational.org/art...ing/index.html

I agree with Cataholic that it sounds behavioral.

Since I don't remember your posts from July, can you repeat/share some info with us so we can help?

How many litter boxes do you have?

What type of litter? Do they all use the same litter? Do you use scented litter? I've met quite a few people that tell me their cat will only use XXX type of litter (fill in the blank..."Yesterday's News" or "Pine" "Sweatscoop" or clay...)

What depth of litter? Are all litter boxes the same? (One of my fosters hated it more than 1" deep because she couldn't stand the sinking feeling beneath her feet. My two girls now prefer to build sand castles )

Have you tried separating the litter boxes so someone could have a little privacy?

Is it possible one of your cats is harassing the others while using the litter box?

Are they located near anything noisy or scary?

I apologize if you've already been through this, but it sounds like you need to make sure you've covered everything.

Finally, are you willing to consider "drastic" litter box actions? My little RB Quasar, after some stress in her life, would only urinate on carpeting. The vet said some cats just prefer the way it feels. Anyway, we gave her cut up carpet squares in the litterbox to solve the problem. Of course, I had to constantly launder the carpet squares, but the rest of the house was safe.

We just need to figure out what's going on inside your cats' heads!