As a part of the study on the composition and development of microbial mats, it was attempted to develop new more effec-tive methods for the isolation and purification of cyanobacteria. The difficulty often encountered in the isolation and obtaining of axenic cultures is commonly known and well reflected by the great variety of purification methods used. Although numerous techniques have been described, most of them either failed or were not suitable for certain types of cultures. Differential gel electrophoresis and Percoll gradient centrifugation, applied by us, appear to be more effective than some other methods in reducing the number of heterotrophic bacterial 'contaminants' present in freshly isolated cultures of cyanobacteria from marine environment.