The unarmoured dinoflagellate,Gymnodinium breve(orPtychodiscus brevis) was used to test the effectiveness of the photodynamic action of hematoporphyrin. Four systems were compared: light control and dark control (organisms in light and dark, respectively, for 24 hours)versusdark and light test (organisms plus 8 x 10‐5Mporphyrin in dark and light, respectively, for 24 hours). Essentially total destruction was observed for the last system (light test) and no statistically significant differences were observed among the other three systems. The effect of concentration of porphyrin was tested in preliminary experiments using the (10‐4.1, 10‐5.1, 10‐6.1, 10‐7.lM) and the lowest concentration had no discernable effect.