Growth optimization of a docosahexaeneic acid (DHA) producing marine fungus, Thraustochytrium aureum (ATCC 34304), resulted in cell productivity of 5.7 g/l and 460 mg/l of total lipid in 69 h flask culture, almost twice the level of cell productivity obtained for the strain reported previously. The lipid extracted from the cells was primarily composed of triacylglycerol and contained about 40% DHA. The fermenter culture showed a lower growth rate than the flask culture indicating growth inhibition due to mechanical stirring. Cells grown in the fermenter showed a coagulant tendency. ATCC 34304, in culture, seems to reproduce asexually and the life cycle of the cells consists of a zoospore stage, trophic cell stage and sporangium stage. Zoospore release was observed once during the early growth phase throughout growth period both in flask and fermenter culture.