It's fine. I know the standard recommendation is 2 sets of true leaves but that is just for convenience of handling. You can do it at the first true leaves stage too.

And those of us that germinate 500 seedlings in 1 flat often prick them out and pot them up at the cotyledon stage when they still only have one tiny tap root. I just about 200 of them today. Of course at that point they go into 1" cells rather than a 16 oz cup. ;)

It's a bit more risky so takes more care but once you get the knack down it is easy and quick to do.

Dave

digdirt...I have been transplanting at the cotyledon stage too, but I only started doing that last week so I don't know yet if I have been successful. Nothing has died yet. I put them in a 1020 flat with 36 cells. Do you think that's ok? anything else I should be careful about? I really love not potting up the plants until I know they've germinated.

The 36 cell are awfully big for cotyledons. Got anything smaller? 72 cell size is perfect at that size.

If not just take care with watering as a cell that size will hold a lot more moisture than they can handle right now. Use the "stick in your finger" test before watering. And transplant them deep - to just below the cotyledons.

If nothing smaller an alternative would be to use the 36 cells but only 1/2 full and then fill up as they grow.

Dave