A study was made on the toxicity of polymyxin B to two blue-green algae. Above a critical cell density, the toxicity level of polymyxin related to cell density ofAnacystis nidulansandChlorogloea fritschiiwas approximately linear. Below this the toxicity level was almost independent of cell density. Polymyxin was much more toxic toA.nidulansin the light than in the dark. Contrary to what might be expected from the known properties of polymyxin, no interference with polymyxin toxicity by any cation could be detected.