A biomass is provided which consists essentially of cells of dinoflagellates of the class Dinophyceae which have been cultivated to produce high cell mass and high DHA content. Biomass consisting of cells of Crypthecodinium cohnii is preferred. The cells are cultivated in an aerobic fermentor to achieve a cell density of a least 10 grams biomass per liter of nutrient solution by maintaining in the fermentor a dissolved oxygen level of at least about 10% of air saturation, and by inducing a stationary phase with a limiting nitrogen source in the nutrient solution. Under these conditions a single cell oil which contains at least 20% docosahexaenoic acid (DHA) in the triglyceride form at a concentration of at least about 1.5 grams per liter of nutrient solution is produced in the biomass. The biomass is suitable as an aquaculture feed.