The cyanobacterial genus Schizothrix is traditionally classified in the special family Schizotrichaceae (order Oscillatoriales) according to the structure of the filaments and thallus: one, two, or more ensheathed and fasciculated trichomes are enveloped by common sheaths. The fine structure of cells and filaments of two natural populations of typical Schizothrix-species (S. facilis, S. lacustris) were investigated in our study. The ultrastructure of trichomes was found to be similar to the pseudanabaenacean types (thylakoid arrangement, inclusions, cell wall), and indicates the close relationship to this group of simple filamentous cyanobacteria. The special life form, which was considered as the most important phenotypic intergeneric (and interfamilial) differentiating character was proven: Fasciculated trichomes are enveloped by their own sheaths, and form (usually heteropolar) filaments enveloped by another common sheath. However, in spite of the fact that the ultrastructure and morphology of trichomes were found to be similar to other pseudanabaenacean types, the relationship to Pseudanabaenaceae must await detailed molecular studies to be more completely evaluated. The first molecular results concerning a few Schizothrix-like strains from Antarctica show that most belong to a cluster, which is separated from the other oscillatorian clusters. This could support the genetic basis of the Schizothrix genus.