- The effect of algal density on cell growth and composition with special reference to the production of eicosapentaenoic acid (EPA) and gamma-linolenic acid (GLA) was investigated in semi-continuous outdoor cultures of Monodus subterraneus and Spirulina platensis. In both species, an exponential decrease in specific growth rate and a positive skewed pattern in biomass productivity were associated with an increase in density. In M. subterraneus, the highest EPA cell content (38% of dry weight) occurred at the optimal density of c. 4 g l(-1), i.e. the density that yields the highest output rate of biomass per culture volume or area. In S. platensis, high density was associated with a decrease in total fatty acid content, but the relative abundance of GLA increased. GLA content was therefore fairly stable at c. 1% of dry weight throughout the range of algal density tested. Fatty acid desaturation seemed to be associated with an increase in culture density of S, platensis, resulting in an increase in the proportion of the fatty acids 16:1 and GLA and a decrease in 16:0 and 18:2. Using a flat plate reactor with a narrow light-path and intensive stirring which facilitated high cell concentration, we obtained maximal EPA productivity of 58.9 mg l(-1) day(-1) in M. subterraneus and maximal GLA productivity of 26.4 mg l(-1) day(-1) in S. platensis. The former is the highest EPA production rate reported to date for any microalga, and the latter is the first report on GLA productivity by microalgae. The data presented herein suggest that maintaining very high, yet optimal culture densities (i.e. 4-10 g l(-1)) in enclosed reactors represents an efficient operating mode for enhancing productivity of desired polyunsaturated fatty acids.