The heterothallic strains of Cosmarium botrytis var. subtumidum Wittr. provide the first material for genetic investigations in the desmids. As a result of the survival of only two non-sister-nuclei following the second division in meiosis, two genes rather than four are produced by a germinating zygospore. The mating type of sexual strains is shown to be determined by a single gene, which segregates in the normal Mendelian fashion. The two mating types segregate, as a rule, in the first division of the meiotic process; therefore, the two genes from a single zygospore are usually of different mating types. The abortion of all zygotes resulting from certain back- crosses of genes with the minus parental strain is due to a single factor which is undetectable in the haploid vegetative cell or in the heterozygous condition in the zygote. Zygotes formed by the fusion of two gametes, each carrying the lethal factor, always lyse shortly after their formation. The lethal factor is inherited as a single gene, segregating, as a rule, at the first division in meiosis. There appears to be no linkage between the genes for mating type and lethality.