The triangle sail mussel (Hyriopsis cumingii Lea) is the most important mussel species exploited for commercial freshwater pearl production in China. However, little is known about the genes that arc involved in pearl formation. A complementary DNA library from mantle tissue of Hyriopsis cumingii was constructed to identify the genes required for pearl formation. A total of 5,019 expressed sequence tags (ESTs) were sequenced, which resulted in the identification of 620 contigs and 1,151 singletons. BLAST analysis showed that nearly half (44.7%) of these unigenes were homologous to the known genes and 55.3% were genes with unknown functions. Based on the sequence similarities, 29 genes were putatively involved in pearl formation. These EST data provide the basis for future studies of the molecular mechanisms underlying pearl formation.