Intergeneric transfer of streptomycin-resistance marker from a unicellular blue-green algaAnacystis nidulans to a filamentous blue-green algaAnabaena doliolum was demonstrated. Mutants ofA. nidulans resistant to streptomycin, occurring spontaneously or mutagenically-induced could be isolated easily. Naturally occurring streptomycin-resistant mutants ofA. doliolum could not be detected. Attempts at isolating such mutants either in nitrogen-free medium or in nitrate containing medium were unsuccessful. However, a streptomycin-resistant strain (recombinant) ofA. doliolum could be isolated in a mixed culture of streptomycin-sensitiveA. doliolum and streptomycin-resistantA. nidulans.