Nine strains of diatoms (representing four species) were screened for their ability to produce eicosapentaenoic acid (EPA) when cultured heterotrophically on glucose. Four strains were able to produce EPA heterotrophically using glucose as its carbon and energy source. Of the four, Nitzschia laevis was the best EPA producer, yielding 0.017 g g(-1) dry cell weight. N. laevis was the only species tested which synthesised more EPA heterotrophically than photosynthetically. This study shows that N. laevis is a potential source of EPA production using heterotrophic culture conditions with glucose as the carbon and energy substrate.