The study aimed to survey the flora of the Cosmarium genera on a tributary of Itaipu dam in western Paraná State. Samples were collected monthly from the biological community in five sampling stations in the limnetic region of the São Francisco Falso river tributary, Santa Helena, Paraná, between July 2003 and June 2004. Were used as criteria of occurrence of taxa in the samples constancies accidental, incidental and constant. The inventory resulted in the identification, description and illustration of 23 taxa, including 10 at the specific,13 infraspecific level, and of these, seven varieties typical of their species and six varieties not typical of their species, and taxonomic two forms. The vast majority of taxa surveyed (69.5%) had an accidental constancy, and the remaining species (30.5%) represented by the constant accessory. There wasn't occurrence of constant taxa.
desmids; São Francisco Falso River; taxonomy
