Social structure and interactions between the anemonefishes, Amphiprion clarkii and A. perideraion, which utilize the same host sea anemone Radianthus kuekenthali, were investigated on a coral reef of Okinawa Islands, Japan. In an 87 × 373 m2 study area, 98 sea anemones were inhabited by both species (32.5%), by only A. clarkii (48.9%), or by only A. perideraion (18.6%). A group of A. clarkii often occupied two or more individual hosts, and group members often interchanged. However, a group of A. perideraion usually used only one host and migration between groups was rare. The larger A. clarkii suppressed reproduction of A. perideraion in cohabiting groups, while A. perideraion suppressed settlement of Juvenile A. clarkii to its own hosts. Juvenile A. clarkii settled on small hosts as well as on large hosts, whereas juvenile A. perideraion settled only on large hosts. Coexistence appears to be possible in part by differences in settlement patterns between juveniles of the two anemonefishes.