Looks exhausted from the heat, what's the temp like, looks like some water will bring it back to life, does not look dead.

unfortunately, despite watering it every day since I took that picture on Wed., it's much worse looking now. This is what happened to all my seedlings that I planted. I kept them well watered. We have extreme heat right now, but when the others died, it wasn't that hot.

"despite watering it every day since I took that picture on Wed"

...and now you have your answer. Over-watering. Roots need oxygen as well as water.

It's common in high heat conditions from noon-5pm when the sun is beating down for leaves to look like crap. If you want to know if it's just high heat or it needs water, wait until the sun goes down a bit. If they're still limp once the sun is down it probably does need water.

You can also cut down on how much water evaporates out your soil putting some much on top of your containers...keeps the roots cooler, too.

Thank you for the responses. I'm still not convinced it was a water or heat issue, since all the peppers I bought were watered the same, and experience the same heat. I finally pulled out this pepper, it was definitely dead. The roots were very short, and sparse. This is how all the other peppers that died (that I grew from seed) were when I pulled them up. Is it possible that the roots didn't grow enough because I didn't transplant them soon enough? Why did the roots not grow after transplanting to the permanent location?

That is classic over-watering. That pot is too big for that size plant especially if you are watering it everyday. If the root ball was still very small when you pulled it out that most likely means that the soil around it was too saturated for the roots to move through. Happens to me every year with some plants.

In the extreme heat we have all been having our plants will wilt during the day. That does not mean that they need to be watered, it is simply a way for the plant to stop transpiration and water loss from its leaves. I go outside after the sun has begun to set to see if any plants are still drooping. And do not confuse the folding down of stems and leaves from the dark for a plant needing to be watered. Many species will droop at night when there is no light for them to bathe in.

Were they all growing in that peaty, water-retentive mix?
Sounds like the roots were staying too wet. That's a large volume of mix for a small plant.

Josh

I don't disagree with the possibility of overwatering / soil mix staying too wet down in the pot for the roots, but, I am surprised that the leaves aren't more yellow because of that.

I had to pot down a couple because the plants were not growing and yellowing due to too big of a pot / too wet.

There may be something else happening that is contributing to the problem.

OK, that make sense. The seedling wasn't big enough for the pot when I transplanted. The soil is pro-mix, plus fertilizer. But, this also happened to many of the plants I had in the ground. I started my pepper seedlings in late Feb. and since they didn't have more than 1 set of true leaves, I didn't transplant to bigger pots (out of 6-pack cells) until the end of April. when I did transplant, I was surprised how much root system there was in the cells, compared to the size of the plant. So, next year I think I'll start the peppers earlier in Feb., and transplant to bigger pots much sooner. I'd like to have bigger plants before they go outside to their permanent homes next year. It's so expensive to buy pepper plants and so many varieties are hard to find, I really want to figure out how to be successful from seed myself!