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This paper refers to a case of polymorphism in the desmid genus Xanthidium Ehr. It is based on material from Lake Dois Irmãos, the main body of water in the Zoological and Botanical Garden in Recife, Pernambuco, northeastern Brazil, collected at 4 different times of the 3 year during 1967 and 1968. A detailed examination of almost 1300 specimens showed an enormous variety in form of Xanthidium regulare Nordst., X. fragile Borge, and X. pseudoregulare Borge, thus allowing the authors, to draw the following conclusions: (1) the name X. regulare Nordst. should be retained until further and more detailed studies on form variation within the species are available; (2) the names X. regulare Nordst. var. asteptum Nordst. in Borge, X. regulate Nordst. var. sexangulare Grönbl., X. regulare Nordst. var. sexangulare Gröbl. f. robustior Gröbl., X. fragile Borge, X. fragile Borge forma, and X. fragile Borge var. depauperatum Borge should be considered synonymous, all referring to a single variety of X. regulare Nordst., var. asteptum Nordst. in Borge emend. C. Bic. & L. M. Carv.; (3) X. pseudoregulare Borge must be treated as a variety of X. regulare Nordst. and must be called X. regulare Nordst. var. pseudoregulare (Borge) C. Bic.& L. M. Carv. Finally, a key is given to the 3 varieties of X. regulare Nordst. proposed in the present paper. [ABSTRACT FROM AUTHOR]