Dark fermentation in the marine green alga, Chlorococcum littorale, was investigated with emphasis on ethanol production. Under dark anaerobic conditions, 27% of cellular starch was consumed within 24 h at 25 degrees C, the cellular starch decomposition being accelerated at higher temperatures. Ethanol, acetate, hydrogen and carbon dioxide were obtained as fermentation products. The maximum productivity of ethanol was 450 mu mol/g-dry wt. at 30 degrees C. The fermentation pathway for cellular starch was proposed from the yields of the end-products and the determined enzyme activities. Ethanol was formed from pyruvate by pyruvate decarboxylase and alcohol dehydrogenase. The change in fermentation pattern that varied with cell concentration in the reaction vials suggested that the hydrogen partial pressure affected the consumption mode of reducing equivalents under dark fermentation. Ethanol productivity was improved by adding methyl viologen, while hydrogen production decreased.