Abstract The morphology, ecology, life cycles, ultrastructure and sequences of the 16S rRNA gene of four cyanobacteria strains from different localities putatively identified as Mastigocladus laminosus were studied. Our strains fall into three separate clades, and conclusions based on “classical” taxonomical methods correspond with conclusions based on sequences of the 16S rRNA gene. According to all methods used, M. laminosus sensu stricto includes true branching strains from thermal springs only, and the genus Mastigocladus is separated from all soil species in the genus Fischerella. The genera Mastigocladus and Fischerella have a confused taxonomic history because the generitype of both taxa was collected from Karlovy Vary Hot Spring within 15 years of the description of each. Strains with other ecological (nonthermal locality) or morphological characteristics (thermal habitat but unbranched forms) belong to different genera. We begin the revision process by moving Mastigocladus laminosus var. indica t...