The amino acid sequences of the human terminal complement components show extensive structural similarity to each other. In this study the C8beta and C9 cDNAs of Japanese flounder, Paralichthys olivaceus, were cloned and analyzed. The derived deduced amino acid sequences of the two terminal components were homologous to those of humans, in that the sequences of both species contained LDL receptor, EGF precursor, and two thrombospondin domains. Japanese flounder C9 was found to have a second thrombospondin region in the C-terminus, similar to that reported for rainbow trout and pufferfish. Moreover, these two complement component cDNAs of Japanese flounder had partial similarity to human perforin. These findings show that Japanese flounder C8beta and C9 have similar structures, which supports the hypothesis that the terminal complement genes originated from the same ancestral gene. Collectively, these features emphasize the strong similarity among the members of the terminal complement family.