The serpulid annelid Ficopomatus enigmaticus is found as a non-indigenous species in many subtropical and temperate habitats, where it often has major effects on the physical structure and community ecology of invaded habitats. In the northeastern Pacific, it has been present in northern California since about 1920, but clearly established populations have not previously been reported from southern California. We describe a large population of F. enigmaticus in the intertidal zone of the Los Angeles River, near Long Beach, California, and a much smaller population in the nearby Port of Los Angeles. Both reproductive adults and new recruits were common in the Los Angeles River population, suggesting that it is well established. We also describe previously unpublished observations of two additional populations in isolated lagoons in Santa Barbara County. Broader surveys aimed at establishing the distribution of this serpulid in central and southern California are needed to evaluate hypotheses on its pathway and timing of introduction to southern California, to evaluate risks of intraregional spread, and to begin to explore management strategies. Key words: biological invasion, exotic species, Los Angeles River, polychaete
