My son did the same thing. This summer, I switched him to undies, and he had no problem peeing in the potty, but he would wait until wearing his nap time diaper to poop. He also used to cry and scream if I tried putting him on the potty. But, we now have pooping success (yay!!!!) so hopefully you will soon, too! What worked for us was waiting till he was ready and then using a step-by-step approach. At 3.5, she's probably physically ready, just scared, but doing it step-by-step may ease that. I think the fact that she is waiting until nap time will actually help you ... it means she can poop on demand and that her body is on a schedule. I might start by trying to have her go (in her pull up, however she's comfortable) before nap (as opposed to during / after). Tell her she should try to go now so that her tummy feels better and so she can sleep better. Once she's used to doing that, then have her poop (again, in a pull up, in what ever position is comfortable) in the bathroom. (Important: tell her about this step ahead of time, to reduce resistance ... emphasize she doesn't have to go on the potty yet). Then, once she does this well for several days, sit her on the potty IN HER PULLUP to poop. This will be the hard step, because she'll have to go in a different position, but letting her wear the pullup will ease the transition and give her more confidence. I would chose a day when you don't have to go anywhere and when you can focus on your daughter without losing patience. This was hard for my son, and we spent 3 days going back and forth to the potty as he kept saying ("I need to poop! ... then ... "I can't!). Finally, on the third day without pooping, I told him to just go ahead and poop standing up and that then I would put him on the potty. He said "No ... no ... I want to poop standing" and I said "Don't worry about it" and once he started (his face flushes, so its easy to tell) I put him on the potty. For the first several days, that's what we needed to do .... he would 'get started' standing, then I'd transfer him and he'd try to continue, at first without success, then with more. Eventually, he found a comfortable position, and he was able to poop (in a diaper) on the potty comfortably. This took a week and a half. Once I could tell he was comfortable with that step, I told him that he was ready to poop without a diaper (I always tell him of changes well in advance to minimize resistance). I was expecting more drama, but there was none! Now that he knew a comfortable position, it was easy. From that first time, pooping on the potty comfortably, he was trained! I really think letting him use the diaper on the potty eased the transition for him. A few other things we did: 1) I gave him 'pooping medicine' (a bit of flaxseed oil, mixed with jam), both to keep his stool somewhat soft and as a placebo ... to make him think it would be easier to poop, and 2) small rewards (e.g. a gummy bear) and LOTS of praise each time he pooped the 'correct' (whatever step he was on) way. My son can go through a whole list of who's proud of him, and by now he's added everyone he knows, including our pets and his baby brother. :-) Good luck ... and keep in mind ... everyone learns eventually! It will happen!