The reproductive behavior of Pomacentrus nagasakiensis Tanaka was studied on the island of Miyake-jima, Japan, from July 1973 to October 1975.Reproductive activity began in May, when water temperatures reached 22°C and continued until early September.The heaviest concentrations of reproductive activity coincided with the waxing of the moon, reaching a peak 2-3 days prior to the full moon.<BR>Like other pomacentrids, males construct and guard nests.Smooth sides of boulders or discarded pots, pipes, tires, etc.are used as nesting surfaces.Females are attracted with typical pomacentrid “signal jumps”.Females are led to nests by an enticement display, which is clearly a distinctive action pattern from the signal jump.Spawning usually occurs in early morning or late afternoon.Several thousand eggs are laid in a single spawning.The male guards the nest and fans the eggs.Typically, a male may guard about ten patches of eggs in a season.Spawning with two or three females may occur within a few days, and a single male may care for three clutches of eggs in a single egg patch.<BR>Males with nests in favorable locations defend their territories persistently throughout the breeding season.Territories in less favorable locations are defended only during periods of active spawning.<BR>The following motor patterns are recognized for P.nagasakiensis during the courtship phase of reproduction: signal jumps, enticement, chasing, and skimming.