Fertile cultures of eight species of desmids have been established. Six are homothallic, one is heterothallic and one has homothallic and heterothallic varieties. Two incompatible pairs of mating types have been found in the last species (Cosmarium botrytis). Incompatibility and isolation of mating types in natural habitats may be responsible for the apparent sterility of many species of desmids. Conjugation between two morphologically distinct varieties has been induced. The inheritance of mating type has been studied. A lethal factor affecting this inheritance has been noted in one species. Mechanisms for the inheritance of the lethal factor are suggested.