Two cases of haplosporidian infection occurred during 1993 in Pacific oysters Crassostrea gigas from the French Atlantic coast. The localization and ultrastructure of the plasmodia are described. In situ hybridization of infected tissue sections was conducted with DNA probes for oyster-infecting haplosporidians. The Haplosporidium nelsoni-specific DNA probe MSX1347 hybridized with the C. gigas parasite, and the H. costale-specific probe SSO1318 did not hybridize. Total genomic DNA was extracted from the infected tissue sections for polymerase chain reaction (PCR) amplification of the haplosporidian. PCR amplifications with H. nelsoni-specific primers and with universal actin primers did not yield the expected products of 573 and 700 bp, respectively. A series of primers was designed to amplify short regions of small subunit ribosomal DNA (SSU rDNA) from most haplosporidians. The primers encompass a highly variable region of the SSU rDNA and did not amplify oyster DNA. PCR amplification of the infected C. gigas genomic DNA with these primers yielded the expected-sized product from the primer pair targeting the shortest region (94 bp). This PCR product was sequenced and it was identical to the corresponding SSU rDNA region of H. nelsoni..