Previously it has been shown that green microalga Chlamydomonas reinhardtii is capable of prolonged H-2 photoproduction when deprived of sulfur. In addition to sulfur deprivation (-S), sustained H-2 photoproduction in C. reinhardtii cultures can be achieved under phosphorus-deprived (-P) conditions. Similar to sulfur deprivation, phosphorus deprivation limits O-2 evolving activity in algal cells and causes other metabolic changes that are favorable for H-2 photoproduction. Although significant advances in H-2 photoproduction have recently been realized in fresh water microalgae, relatively few studies have focused on H-2 production in marine green microalgae. In the present study phosphorus deprivation was applied for hydrogen production in marine green microalgae Chlorella sp., where sulfur deprivation is impossible due to a high concentration of sulfates in the sea water. Since resources of fresh water on earth are limited, the possibility of hydrogen production in seawater is more attractive. In order to achieve H-2 photoproduction in P-deprived marine green microalgae Chlorella sp., the dilution approach was applied. Cultures diluted to about 0.5-1.8 mg Chl center dot L-1 in the beginning of P-deprivation were able to establish anaerobiosis, after the initial growth period, where cells utilize intracellular phosphorus, with subsequent transition to H-2 photoproduction stage. It appears that marine microalgae during P-deprivation passed the same stages of adaptation as fresh water microalgae. The presence of inorganic carbon was essential for starch accumulation and subsequent hydrogen production by microalgae. The H-2 accumulation was up to 40 mL H-2 gas per 1iter of the culture, which is comparable to that obtained in P-deprived C. reinhardtii culture.