Chromosomal DNA from nine species of filamentous cyanobacteria as diverse as Nostoc, Gloeotrichia and Plectonema is suggested to be extensively modified (methylated) by its resistance to cleavage by a number of restriction endonucleases. A remarkably similar pattern of DNA modification in these species contrasts with the known heterogeneity of their type II restriction endonuclease content. In particular, Nostoc PCC 73102, which lacks detectable sequence-specific endonucleases, is shown to possess extensive DNA modification. The use of isoschizomers demonstrates the presence of a methylase in the filamentous strains analogous to the dam enzyme of Escherichia coli. As a preliminary to assessing the significance of the DNA modification, a study of susceptibility to restriction endonuclease cleavage of the genomes of five unicellular cyanobacteria revealed considerable variation between the different strains. The significance of the DNA modification patterns elucidated is discussed in terms of the restriction endonuclease content and cellular differentiation of the relevant cyanobacterial strains.