Two prominent diatoms encountered in oyster-ponds,Haslea ostreariaand Skeletonema costatum,were grown in batch and in a semi-continuous modeunder light of different spectral quality, white, blueor blue-green. The last corresponded to white lightmodified by a water-soluble pigment, marennine,produced by H. ostrearia. After acclimation tothe different light treatments, the growth rates ofboth species showed little variation with respect tolight quality. The parameters for photosynthesisvsirradiance curves were very similar in H. ostreariagrown under the three light conditions,whereas S. costatumthe maximum photosyntheticcapacity (on a chlorophyll abasis) wassignificantly reduced under blue-green light. Fluorescence analyses confirmed the data forphotosynthesis, with the operational fluorescenceyield decreasing faster with increasing irradiance inS. costatumgrown under blue-green light. InH. ostrearia, fluorescence yields undersaturating irradiance were closely similar in thethree light conditions. The results are discussed inrelation with the prominent development of H.ostreariathat can outcompete other diatoms inoyster-ponds.