SUMMARYIn axenicChlorella pyrenoidosaChick cultures, extracellular release was linear with time, but plateau‐type curves were obtained in cultures with added bacteria. Initial rates of excretion were identical in both, systems. Kinetics of extracellular release in axenicAnabaena flos‐aquae(Lyng.) Bréb. cultures were more complex than inChlorellabut the initial excretion rates were identical in axenic and mixed algal‐bacterial cultures. In lakewater, extracellular release kinetics resemble the pattern in mixedChlorella‐bacteria cultures. An explanation is an initial lag in bacterial utilization of algal extracellular products. As a result, bothin situand in the laboratory, consecutive short, experiments give higher excretion rates than single long incubations. It is suggested that the former are close to total or gross extracellular release rates whereas the latter give net values, detecting only substances not, removed by hete