This work attempts to identify growth conditions for maximal productivity of cell mass and of eicosapentaenoic acid (EPA) in ultrahigh cell density cultures of Nannochloropsis sp. Using flat plate reactors with a narrow (1–2 cm) light path and rigorous stirring exposed to high photon flux densities (1000–3000 μmol photons m−2 s−1), record population densities (1·2–1·4×1010 cells ml−1) were obtained. Consequently, the EPA content of the culture (mg l−1) was higher by some two orders of magnitude than reported hitherto for cultures of much lower cell concentrations. In continuous cultures, highest culture EPA yield coincided with maximal output rate of cell mass. The very high population densities and output rates of cell mass and of culture EPA were possible provided culture medium was replaced at least every 48 h. Inhibitory activity, for which a bioassay was developed, was thereby removed. When nutrients were added frequently in order to prevent nutrient limitation without removing the inhibitory activity in the cultures, cell proliferation ceased after reaching some 30% of the attainable maximal cell number, and the culture gradually deteriorated. Inhibitor-induced culture deterioration was fully reversible when the growth medium was replaced with fresh medium.