Abstract: Haplosporidiosis is a serious mollusc disease caused by parasitic Haplosporidium. The Haplosporidium-specific PCR detecting method, recommanded by World Animal Health Organization (OIE), was adopted to investigate Haplosporidiosis prevalence of oyster in three sampling sites of China east bays. Results showed that there was Haplosporidium prevalence in all the four sites in different degree. Through combining with DNA sequencing and sequence analysis, the infection species was identified to be Haplosporidium nelsoni.

Key words: Haplosporidium; PCR; sequencing; prevalence; sequence analysis 