By treatingSynechococcus elongatusandNostocsp. with NTG, a large spectrum of morphological and pigment mutants have been obtained. The main mutants are those involving long, nonseptate and branched filaments inSynechococcus, ramification, distortion, enlargement of cells, several terminal heterokysts, yellow mutant and lack of necridium differentiation inNostocas well as anastomosis of terminal or intercalary cells inNostoc.Some evolutionary aspect are also discussed.