Parasites have been observed to affect all major algal classes, but few detailed reports of parasitic infection of dinoflagellates exist. By causing lysis of host cells, parasites may influence the population of the host. As an example, many Alexandrium minutum cells have been found to be infected by an unknown parasite after a bloom in the estuaries of northern Brittany, France. The new parasite is probably similar to the new genus Parvilucifera belonging in the Apicomplexan complex, recently described from scandinavian waters. The release of the biflagellate zoospores was video recorded under light microscopy, and the sporocyst structure was examined with the scanning electron microscope. Direct preparations of the zoospores, examined with the transmission electron microscope, show typical hair ornamentation on the long flagellum, the short one being naked and acronemated. The parasite was found to infect laboratory cultures of several other dinoflagellate species but the contamination appeared to be restricted to this taxonomic group as diatoms and a raphidophyte strain were not infected. Estimates of parasite-induced mortality indicate that it is capable of removing a significant fraction of dinoflagellate biomass in a short time. The same parasite was found again in natural samples during summer 1999, but the effect of this parasite on natural populations remains to be estimated.