Molecular genetic keys for identification of 3 commercially cultured oysters (Crassostrea belcheri, Crassostrea iredalei, and Saccostrea cucullata) in Thailand were developed based on restriction analysis of 18S ribosomal DNA and cytochrome oxidase subunit I (COI). Digestion of the amplified 18S rDNA with Hinf I unambiguously differentiated Crassostrea oysters from Saccostrea oysters and Striostrea (Parastriostrea) mytiloides. In addition, species-specific restriction fragment length polymorphism patterns of C. belcheri, C. iredalei, and S. cucullata were consistently observed when the gel-eluted COI was digested with Mbo I and Dde I. Thirty composite haplotypes were observed across all individuals. Species-specific composite haplotypes were found in C. belcheri (AAAA and AAAB), C. iredalei (AABC and AABU), and S. cucullata (BBCD and BBCE), respectively. The most common composite haplotype of COI in C. belcheri (AAAA), C. iredalei (AABC), and S. cucullata (BBCD) was amplified, cloned, and sequenced. Detection of C. belcheri and C. iredalei based on polymerase chain reaction was further developed using more specific primers (HCO2198 and R372) followed by digestion of a 372-bp product with Mbo I.