Microalgal population of oyster ponds is dominated by several tychobenthic diatoms which provide a very good food to grow oysters. The mechanisms which allow them to dominate have been put under research. H.P.L.C. analysis demonstrated 17 amino acids amounted 10-17% of total DON, i.e. 0.6-11.5 mu g-at/l nitrogen. Serin and glycin were the most abundant. None of the substances was related to the presence of oyster. Glutamic acid, alanin and ornithin concentrations showed the greatest variations; they are suspected to play the major role as nutrient source. Screening of nutritional capabilities of ten local isolates of micro-algae to use 42 organic substances as sole source of nitrogen, demonstrated urea, glutamic acid and hypoxanthin are assimilated by all the ten algae, yet urea alone always supports a growth as good as the one showed with NO sub(3). Other good sources are guanin, ornithin and glycin. Most of the substances, however, are taken up and assimilated by a few strains and subsequent growth is light.