If you planted that from saved sedd and if you had nothing like that last year then I venture to say that it's a cross pollination of some sort. Did you have yellow squash like butternut and pumpkins. Looks like that pumpkin is a cross between the 2. It would be interesting to see what it tastes like. You might have a great new variety of pumpkin on your hands or it might taste awful--it's hard to say.

oilpainter--Thanks for your input. I took the seeds for this plant from pumpkin/squash that was used in a window display last year at my work. Honestly, the girl that bought the display stuff got them mostly because some of them were ornamental. I never thought about taking a photo of each squash before I tore into them for their seeds. It sure would have made it easier to ID them now. One other type of seed is growing too, but seems to be way behind the 'yellow pumpkin'...and looks differently starting out. The yellow pumpkins are about soccer ball size right now. When do I pick these mysterious pumpkins/squash?

Beats me when you'd pick them!

I was thinking cross pollination in a garden.

Have you done a google search for novelty pumpkins? I know there is a white one. Maybe that's what you've got and it has to change color. Other than that I think you just have to wait and see what happens--interesting!

Same rules as for any winter squash. When the skin is hard and the stem is dry and hard. Usually it is best to wait until the vine starts dying. Concur that it is likely a cross breed. However there are orange pumpkins that start out yellow.

they look just like the pumpkins I am growing, that are taking over my entire back yard.

Alberta

Thanks everyone! I'll be anxious to harvest this mystery squash/pumpkin to see what it tastes like.

albertar---What kind of pumpkin is this that you are growing? Did you plant it? Or was it a volunteer? I wonder if it's the same as mine.

Sizwizz
The seeds were sent to me via a woman here on the forums 2 years back. She had no idea what variety they were but she said she got the seeds from what she termed a "gianormous sized pumpkin". I held the seeds because I didn't have the room to plant them when she first sent them, and this year planted 3 in a hill of pure leaf mulch just to see what would happen. Well what has happened is they have taken over the better part of my back yard, :) the fruit themselves are not ripe yet, but growing daily and very yellow as in your picture. The rinds have not hardened yet, as I did a thumbnail test yesterday, so I guess they have while to go, ;)

Alberta

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Looks like my pumpkins. But not like my pie pumpkins. These were planted from seeds out of a packet.

makete-- Your pumpkins look very much like what I have. So, are they yellow pumpkins? What did your seed packet say about them? Your pumpkins seem a little more round than mine, (could be they grow in random shapes), but otherwise they are the same in color and everything else. Do the bottoms of your pumpkins have the green/yellow striping like mine do in the pictures? Just curious...

Mine do not have that green striping on the botton. Ordered mine from Jungs seeds, think they were called "prize winners".

I don't know Prize Winner, but some of the bragging pumpkins do start off yellow. Big Max is one example. They turn orange when they ripen. According to Jung, Prize Winner gets even bigger than Big Max.