Cross pollination occurs in cucumbers. This means pollen is transferred from a plant of one variety to a plant of another variety. This is done by insects. Although it does not affect the fruit borne this season, if you save the seed and plant them next year, the plants that come from these seeds will be different. So will the fruit. So, if you want to save cucumber seed, plant only one variety. Select strong, healthy cucumber plants and well-formed fruits. Let the fruits hang on the vine until ripe (skin becomes yellowish and hard).
Allow the cucumber fruit to thoroughly ripen on the vine. In fact leave them until the vine is dead. Cut the cucumbers open and remove the seed by squeezing or spooning out the pulp with seeds into a non-metal container such as a drinking glass, glass bowl or jar. Alternately, put water in the container at the beginning of the fermentation process. Set the container aside for one or two days, possible up to five. The pulp and seed covering will ferment so that the seeds can be washed clean with a directed spray of water into the fermented solution. The clean, viable seeds will drop to the bottom of the solution, allowing the sediment to poured off. Several rinsings may be necessary. You can strain the mixture through a colander or flour sieve. Then spread the cucumber seed out on a cloth or paper towel to dry. After seed are dry, package, label and date for storage in a cool (refrigerator), dry location.