This work is part of a survey of the desmid flora in a tributary of the Itaipu Reservoir, in the state of Paraná, Brazil. The objective was to expand the knowledge base regarding the genus Cosmarium (Desmidiaceae, Zygnematophyceae). We conducted a qualitative analysis of 60 samples of phytoplankton, which were collected from along the banks of the São Francisco Falso River between July of 2003 and June of 2004. We identified, drew, and described 18 taxa of Cosmarium. Of those 18 taxa, 8 represent new records for the state of Paraná: C. angulosum Bréb. var. angulosum, C. bireme Nordst. var. bireme, C. clepsydra Nordst. var. clepsydra, C. cf. galeritum Nordst. var. borgei, C. ginzbergeri, C. norimbergense Reinsch var. norimbergense, C. protractum (Nägeli) De Bary var. protractum, and C. rectangulare var. hexagonum (Elfving) G.S. West & West.