Prorocentrum micans Ehrenberg and Gymnodinium splendens Lebour were cultivated in a halfsynthetic medium at 18oc. The generation time was 43.6 ~46.8 ~50.5 h for p. micans and 49.0 ~53.7 ~59.3 h for G. splendens. Variation of nutrient concentrations did not influence the generation time, but caused changes in the production rate of new cells. In the stationary phase, p. micans populations grew by 735 • 50 cells/ml in filtered sea water without addition of nutrients and by 5437 • 1214 cells/ml with the addition of 500 ~g-at NO3-N/I and 30 ~g-at PO4-P/I. In bi-algal cultures, P. micans shows changes in generation time and in cell production, whereas with G. splendens changes occur only in cell production. The degree of change depends on the ratio of the two algae as defined at the start of the experiments (see Table 3). p. micans did not grow in the filtered medium previously used to cultivate G. splendens, whereas G. splendens did grow in the filtrate of p. micans cultures. Presumably G. splendens excretes an inhibiting factor affecting the growth of P. micans, whereas the effect on G. splendens production exerted by P. micans seems to be due to nutrient competition. The relevance of the results obtained from in ~tro experiments on the interpretation of in situ population dynamics is discussed.