To circumvent the co-production of oxygen during hydrogen photoproduction, certain physico-chemical conditions were manipulated. Then, under white light (intensity 5.5 mu mol photon m(-2) s(-1), 18 h dark/6 h light cycle); temperature 27 degrees C; pH 7.5; salinity 25 g l(-1), the marine, filamentous, non-heterocystous cyanobacterium Phormidium valderianum BDU 20041 was found to produce a maximum of 0.2 mu mol hydrogen h(-1) mg(-1) dry wt in the gas phase without oxygen co-production; this was on a par witt? the maximum yield reports in the literature. To compensate for possible reduction in CO2 fixation under these conditions, when 10 mM of different carbohydrates were used, those other than galactose, lactose and trehalose were found to be favourable. For continuous production of hydrogen when amino acids were added as a nitrogen source, isoleucine was found to be the least inhibitory. The organism was found to be essentially a dark hydrogen producer. Copyright (C) 1996 Elsevier Science Ltd.