Random amplified polymorphic DNA (RAPD) marker and sequence analyses of the internal transcribed spacer (ITS) region of ribosomal DNA were used to assess phylogenetic relationships of four Korean oyster species. The average number of species-specific markers identified from five universal rice primers (URPs) by RAPD-PCR was 1.8 for Crassostrea gigas, 3.2 for C. nippona, 3.6 for C. ariakensis, and 4.6 for Ostrea denselamellosa. The length of the ITS (ITS1-5.8S-ITS2) region ranged from 1,001 to 1,206 bp (ITS1, 426-518 bp; 5.8S, 157 bp; and ITS2, 418-536 bp), while the GC content ranged from 55.5-61.1% (ITS1, 56.8-61.8%; 5.8S, 56-57.3%; and ITS2, 54.1-62.2%). A phylogenetic analysis of the oysters based on our RAPD, ITS1, and ITS2 sequence data revealed a close relationship between C. gigas and C. nippona and a distant relationship between the genera Crassostrea and Ostrea. Our results indicated that RAPD and ITS sequence analysis was a useful tool for the elucidation of phylogenetic relationships and for the selection of species-specific markers in Korean oysters.