Never grown them so assuming they are normally green and that one is way over-ripe it should be good for seed saving. Always best to save seeds from more than one fruit regardless of the plant.

Doesn't look like a "tiny fruit" to me. ;-)

Dave

Magic seeds, Dave... turns out they only remain at the ideal size for about 5 minutes (or until you turn your back on them) before they blow up to the size of a football. I've thrown about 50 touchdowns to the cows next door over the past week.

Most cucumbers turn yellow or brown when ripe, so chances are good that the one in the photo has viable seeds. The best thing, though, is to leave them on the plant as long as possible. I am saving seeds for one of my cukes this year, and will cut them off either just before frost, when something takes a bite out of them, or if they begin to show signs of rot. Provided the plants are healthy (it sounds like they are), letting a single fruit ripen on the vine should not stop production.

If the ripe cukes are undamaged, let them cure indoors for a few weeks if possible; the seeds will continue to fatten up inside. Don't discard those that begin to rot... but process those quickly. Carefully cut the cucumber open lengthwise, and scrape out the seeds & as much juice as possible. Ferment the mixture in an open container for several days (much as you would do for tomato seed), watching for the membranes surrounding the seeds to separate. For me, this usually takes 4-5 days.

To clean the seeds, pour the mixture (or part of it) into a tall container, add several times that volume of water (more is better), and the good seeds will sink to the bottom. Some good seeds may float, if their membranes are still attached; beating the floating debris vigorously with a wire whisk should release the membranes, and allow that seed to sink. Carefully pour off the floating debris, refill the container with water, and continue to rinse until there is nothing but clear water, and a layer of seeds in the bottom. Then pour the seeds into a sieve, wick off excess water with paper towels, and spread the seeds out on a smooth surface to dry. To avoid mold, I recommend a fan & frequent stirring for the first day.

I agree with Dave's recommendation about saving seed from more than one plant; it is not strictly necessary, but if you intend to continue saving seeds from this variety, it will help to maintain the health of the population over time.

Oh, by the way... properly dried & stored, cucumber seed has a very long storage life; 5-10 years.