- Abstract Experiments concerning the optimization of growth conditions in the laboratory and mass production of the marine microalga Isochrysis galbana in open reactors are described. In the laboratory, a maximum specific growth rate (μmax) of 0.073 h −1 was obtained, which is equivalent to a doubling time of 9.5 h. In a small reactor (2.5-m 2 pond) the production rate of biomass was 29.5±2 g m −2 day −1 and that of lipids was 6.5±0.5 g m −2 day −1 . In a larger reactor (100-m 2 pond) in which algae were grown first by batch and then by semi-continuous mode, the average output rates of biomass and of lipids were 23.5±1.5 g m −2 day −1 and 5.6±0.5 g m −2 day −1 , respectively. A complete removal of the biomass was achieved in the 100-m 2 pond by means of flocculation with FeCl 3 (180 mg l −1 ) and dissolved air flotation without a prior recycling of part of the effluent. Monoalgal cultures were maintained in the different outdoor experiments throughout the entire 4-month production period.