Cleared lysates of 13 species of filamentous cyanobacteria were examined for the presence of extrachromosomal DNA by using agarose gel electrophoresis and ethidium bromide staining. Seven of the 13 species contained extrachromosomal covalently closed circular DNA, and all but 1 species contained multiple elements. There was no correlation between the presence of extrachomosomal DNA and either the range of metabolic activities found in the cyanobacteria or the differentiated cell types or structures elaborated by the morphologically complex filamentous cyanobacteria.