The round stingray, Urobatis halleri, is a viviparous elasmobranch that inhabits inshore, benthic habitats ranging from the western U.S.A. to Panama. The population genetic structure of this species was inferred with seven polymorphic microsatellite loci in samples collected at three sites in coastal southern California, one near Santa Catalina Island, California and one in the eastern Gulf of California. Urobatis halleri is relatively common, but little is known of its movement patterns or population structure. Small F(ST) values (-0.0017 to 0.0005) suggested little structure among coastal populations of southern and Baja California. The population sampled at Santa Catalina Island, which is separated by a deep-water channel from the coastal sites, however, was significantly divergent (large F(ST), 0.0251) from the other populations, suggesting low connectivity with coastal populations. The Santa Catalina Island population also had the lowest allele richness and lowest average heterozygosity, suggesting recent population bottlenecks in size.