Production of clams is the third most important bivalve production in France and was estimated at 3400 metric tons in 2001 among which 60% came from aquaculture and 40% from fisheries. Surveillance of clam diseases mainly relies on histology and thioglycolate culture, two techniques allowing detection of parasites of the genus Perkinsus but not specific enough to determine the parasite species. Based on a previous study aiming at estimating distribution and prevalence of perkinsosis in France, we selected 34 infected clams from 4 different producing areas in order to characterize the parasite species. Molecular characterization was done by PCR-RFLP targeting the ITS region as described by Abollo et al. (2006). 16 PCR products were cloned and 10 to 20 clones per individual were selected for PCR-RFLP to test potential intra individual polymorphism. 23 clones, including the ones which yielded non expected restriction profiles, were finally selected for sequencing. All the obtained sequences displayed 99 to 100% of homology with Perkinsus olseni confirming the presence of this protozoan in at least four different French marine areas (Golfe du Morbihan, Thau, Leucate, Arcachon). Polymorphism between clones obtained from a same clam or from different clams did not appear geographic dependent.