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This paper refers to a case of polymorphism in the desmid Micrasterias laticeps Nordst. It is based on material collected from an artificial pond in the state of São Paulo, Brazil. A detailed examination of almost 400 specimens permitted the authors to give a detailed description of 13 main morphological forms of the alga and allowed them to draw the following conclusion: (1) For delimitation of genera and infrageneric taxa in desmids it is often necessary to examine sample populations; (2) the morphological characteristics presently used for delimitation of varieties in M. laticeps Nordst. Seem to have no considerable taxonomic significance if isolated individuals are studied; (3) it is necessary to take into consideration Cosmarium arthrodesmiforme Borge and C. arthrodesmiforme var. latius Krieg. & Gerl., as well as M. laticeps Nordst. var. ampliata Krieg., when further studies are undertaken, because they may prove to be merely distinct morphological expressions of M. laticeps Nordst var. laticeps. [ABSTRACT FROM AUTHOR]