In early summer 1998, the parasitic dinoflagellate Amoebophrya sp. Koeppen infected a population of the photosynthetic dinoflagellate Prorocentrum minimum (Pavillard) Schiller in a temperate shallow embayment. This is the first record of such an infection in P. minimum. The parasite was observed live, both within its deformed host as a trophont and outside its host, either as a swimming mass of flagellated cells, the vermiform, or as individual sporonts. Except for the shape of the vermiform, stages of development were in accordance with previous descriptions of Amoebophrya parasitizing photosynthetic dinoflagellates. Planktonic grazers fed on the sporonts. Further studies are needed to decipher the influence of parasitism on the regulation of host abundance.