Major capsid protein (MCP), the adenosine triphosphatase (ATPase), and the PstI fragment genes from five Japanese and three Korean megalocytivirus isolates were sequenced and phylogenetically analyzed with known megalocytiviruses. Phylogenetic trees formed three major clusters (M1, M2, and M3 or P1, P2, and P3), and genogroup I was divided into two minor clusters (M1a/M1b and P1a/P1b) using three target genes. Sequence identity was >97% within each cluster, except cluster II of the PstI fragment (>94% of sequence identity). Interestingly, different genotyping patterns were observed for the same isolates depending on the gene analyzed. The JPN-YelTail and JPN-BfTuna isolates located in the minor M1a cluster, based on MCP and ATPase nucleotide sequences, appeared in the minor P1b cluster based on the PstI fragment, suggesting a shift of phylogenic position in megalocytiviruses. Further study will be conducted to compare the viral antigenicity and pathogenicity between the two isolates showing the shift of phylogenic position and the other isolates clustered within genogroup I.