Over 120 randomly selected strains belonging to 16 genera and to more than 80 species of Desmids were brought into uni-algal culture and subjected to conditions provoking conjugation. Only 3 isolates – all homothallic – exhibited sexual reproduction: Closlerium limneticum Lemn., Cl. moniliferum Bory ex Ralfs, and Micrasterias papillifera Breb. Of Cl. limneticum no zygospores had previously been known. The, generally speaking, poor result of the experiment supports the theory that it is a lack of sexual potential rather than ecological factors which is responsible for the rare incidence of sexual reproduction in Desmids under natural conditions.