ALTHOUGH THE DESMIDS have interested microscopists since the early 19th century, most of our knowledge of the Desmidiaceae consists of descriptions of hundreds of species based on characteristics of the vegetative cells. Zygospores are known for some species, but in only a few have the details of the sexual process been recorded. This lack of knowledge of the sexual process is due mainly to the rather low frequency of its occurrence in nature and the lack of success in isolating and maintaining cultures of desmids in which sexual reproduction could be induced. The first successful efforts to induce sexual reproduction in the Desmidiaceae were those of Klebs (1896). Using natural populations, Klebs was able to induce sexual reproduction in Cosmarium botrytis (Bory) Meneghini by immersing the cells in a sugar solution and in Closterium lunula (Mull.) Nitzsch by increasing the illumination. Repetition of these experiments by Klebs with other strains of the same species proved unsuccessful. Pringsheim (1918) reported successful cultivation of 16 species of desmids. Of these, one Closterium species (C. strigosum?) produced numerous zygotes; but failure of these zygotes to germinate resulted in termination of this strain. In Cylindrocystis brebissonii Meneghini, a member of the Mesotaeniaceae, Pringsheim (l.c.) was able to induce sexual reproduction repeatedly by placing the cells in dist'illed water or in nitrogen-free nutrient solution. Czurda (1926, 1937), Ondraeek (1936), and Kallio (1951, 1953), among others, have successfully cultivated. desmids but none has reported success in inducing sexual reproduction. In May, 1953, a healthy population of Cosmarium botrytis var. subtumidum with numerous zygospores was found in a collection from a small pond near Bloomington, Indiana. Ten vegetative cells were isolated and placed singly in tubes of soil-water medium containing powdered calcium carbonate (Pringsheim, 1950). The tubes containing the individual clones were then illuminated with light of 500+ ft.-c. intensity from a fluorescent fixture in a culture room in which the temperature Was maintained at 21?C. Of the 10 isolations, 6 multiplied so as to fill the tubes partially with macroscopically visible growth, but after three months there was no evidence of sexuality in any of the 6 tubes. Cells from each tube were then mixed with each of the other isolations in all possible combinations. The