AbstractThe oyster cannot synthesize sterols from smaller molecules but must obtain them from its diet, which consists of detritus and small organisms, i.e., mostly single‐celled algae. Algae differ widely in their effectiveness as oyster food. Small (<5 μm) algae which are abundant in sterols and polyunsaturated fatty acids appear to be most effective. Recent studies have shown the occurrence of cholesterol in strains of the unicellular algaeTetraselmis, Chaetoceros andSkeletonema, sometimes in large quantities. In the study reported here, six isolates of a recently constructed algal class, the Eustigmatophyceae, have been examined for sterols and fatty acids by gas chromatography and gas chromatography/mass spectrometry. All strains were shown to contain cholesterol as the principal sterol. Two isolates contained large amounts of total sterol (400–1000 fg/cell), and one (Sticho 0–18) also contained large amounts of eicosapentaenoic acid (20∶5n−3). These biochemical characteristics are desirable in a potential food source for oysters.