Polymorphism in a population of the desmid (Zygnemaphyceae) Cosmarium abbreviatum Raciborski var. minus (West & West) Krieger & Gerloff is reported from material collected from 3 stations at the Lily Pond located in the Parque Estadual das Fontes do Ipiranga, São Paulo City, southern Brazil. From the examination of approximately 200 plants, 6 different morphological expressions were described and the following remarks made: 1) a detailed analysis of sample populations is absolutely necessary for the definition of species and infraspecific categories among desmids; 2) the morphological characteristics presently used for the delimitation of certain species, which are built up of individuals of very small size, seem to have no great taxonomical significance when only isolated individuals are analysed; and 3) taxa, such as Cosmarium angulosum Bréb. var. angulosum f. angulosum, C. abbreviatum Rac. var. minus (West & West) Krieg. & Gerl., C. succisum G.S. West var. jaoi Krieg. & Gerl., C. tinctum Ralfs var. tinctum, and C. minimum West & West var. minimum are of doubtful validity and may prove to be merely polymorphic expressions of C. abbreviatum var. minus.