The reproductive behavior and mating system of the Rock Wrasse (Halichoeres semicinctus) was recorded and monitored around Big Fisherman Cove, Santa Catalina Island, CA from April 2001 to September 2002. Four sites with different relative densities were visited daily throughout the summer spawning season and once a month throughout the spring and fall. Group spawning was the dominant mating system among smaller individuals (<200 mm) with 8–12 individuals in the spawning aggregations. Large (>200 mm) terminal phase males held territories which were 4–5 m² and were observed courting on multiple occasions in an attempt to pair spawn, but only three pair spawns were witnessed during the study. Elaborate courtship displays, including vertical rushes and lateral displays by the males within their territory, preceded pair spawning attempts. Spawning occurred primarily in the morning, but aggressive behavior by the territorial males continued throughout the day. The highest level of both group and pair spawning activity occurred at the highest density site, and only group spawning was seen at other sites. Density transect data showed that ratios of initial phase: terminal phase individuals were skewed toward initial phase in late spring and early fall and were skewed toward terminal phase individuals during peak spawning months (July and August) when many aggregations were present. The overall sex ratio in Rock Wrasse from Santa Catalina Island was nearly 1∶1, and histological data suggest that sex-changed individuals made up 38% of studied males. These data suggest that Rock Wrasse utilize a context-dependent, dualistic mating system that may act to increase opportunities for mating and thus, their overall reproductive output.