Out of 33 strains of unicellular algae examined, H2 evolution was observed only in species of Chlamydomonas, Chlorella and Scenedesmus. While the photoevolution of H2 by these algae was generally stimulated both by an organic substrate and by the uncoupler CCCP(1), response to DCMU varied. On the basis of the response to DCMU, it was concluded that the mechanism of photoevolution of H2 differed from one alga to another. The reaction in some algae appeared to be dependent on either the photooxidation of water or oxidative carbon metabolism for reductant; that in other algae was supported by reductant from both these sources.