Monodus subterraneus is a microalga, which is known for its high eicosapentaenoic acid (EPA; 20:5omega3) content. To produce EPA commercially, high volumetric productivities of microalgae are required. These high productivities can be reached in flat panel photobioreactors with small optical paths that have to be operated at high cell densities (>10 g/L). However, at these cell densities a reduction of productivity is observed. This growth inhibition is probably caused by growth inhibitors released by the microalgae, which have been suggested to be fatty acids. Our aim was to investigate if free fatty acids produced by M. subterraneus inhibited growth of this species. Therefore a bioassay was developed and saturated, unsaturated and poly-unsaturated fatty acids occurring in Monodus were tested on their growth inhibiting properties. Growth of M. subterraneus was completely inhibited at a saturated concentration (96 microM) of palmitoleic acid (16:1omega7). But, the saturated fatty acid palmitic acid (16:0) and the mono-saturated oleic acid (18:1omega9) were much stronger inhibitors. Growth was inhibited for 50% already at concentrations of 0.4 microM 16:0 and 3 microM 18:1omega9, respectively. These fatty acids probably cause the growth inhibition in high cell density cultures of M. subterraneus.