Steinhausia mytilovuln is a globally distributed microsporidian parasite which infects the oocytes of the blue mussels Mytilus edulis and M. galloprovincialis. Despite the intensive monitoring effort made on mussel populations, the parasite has not previously been reported in France. We report herein on the occurrence of S. mytilovum in Mytilus sp. from 1 cultured and 2 natural populations on the northern coast of France, thus extending the parasite's known distribution northwards. We also report on the observation in 1989 of S. mytilovum in M, galloprovincialis from the Golfe de Fos area in the Mediterranean Sea (South of France), S. mytilovum was observed in the European hybrid zone between M. edulis and M. galloprovincialis, which therefore renders the exact taxonomic status of the infected hosts unknown, The prevalence of the parasite was low, which suggests that its effect on mussel populations was probably limited.