Haemolysins are thought to be involved in the ichthyotoxicity of the raphidophyte Fibrocapsa japonica. So far, all haemolytic data were based on the analysis of intracellular haemolysins in concentrated extracts of cells at the end of the exponential or in the stationary growth phase. To gain more insight in the mechanisms of haemolysin production we studied differences in intracellular and extracellular haemolysin activity at various cell densities during exponential growth. The haemolytic activity of F. japonica changed significantly as a function of cell concentration. At increasing cell concentrations, extracellular haemolytic activity decreased while intracellular activity increased. Possibly, the very high extracellular haemolytic activity at low cell concentrations could be caused by growth inhibiting or allelopathic haemolysins.