Mediterranean strains of Prorocentrum minimum do not appear to have the same toxic component as Japanese strains since they showed no cytotoxicity for hepatocytes in culture. However, their toxic components, which appear to block calcium channels, were detectable by the immobilisation test on Diptera larvae. A bio-accumulation experiment in the laboratory showed that the toxins could accumulate in nearly equivalent amounts in the hepatopancreas and meat of cultured mussels. The same toxicity was found in natural samples collected in a period of bloom of P. minimum. These results suggest that P. minimum could be responsible for shellfish toxicity in the natural environment and thus present a risk for human health.