Field experiments were performed to explore the nature of indirect interactions between two groups of ecologically distinct prey that occur on subtidal rocky reefs at Santa Catalina Island, California. Mobile gastropods (Tegula aureotincta, Tegula eiseni, and Astraea undosa) and sessile bivalves (mostly Chama arcana) share a common set of invertebrate predators (lobster Panulirus interrupts, cephalopod Octopus bimaculatus, and whelk Kelletia kelletii). The gastropods, which are secondarily preferred prey, principally occur on cobble reefs and less commonly on high—relief boulder reefs. Sessile bivalves and other species of favored prey are common on high—relief reefs and are rare or absent in cobble areas. The density of each predator species is greatest in high—relief areas containing abundant favored prey. Addition of bivalves to replicate cobble plots resulted in marked increases in predator density, relative to controls, because predators aggregated to areas containing favored prey. Greater densities of predators in the presence of experimentally added bivalves resulted in greater mortality and lower population densities of gastropods relative to controls. Conversely, mortality of Chama was higher where gastropods were common compared with cobble areas containing fewer gastropods. This occurred because the density of predators within a cobble reef, although low overall, was positively correlated with density of gastropods. Thus each group of prey was negatively affected by the presence of the other because each alternative prey increased the local density of predators. Such a doubly negative indirect interaction between prey, mediated by a shared predator, is known as apparent competition; this is the first experimental demonstration of its existence. Because gastropods and bivalves are superior at withstanding predation in different habitats, shared predation and apparent competition may be sufficient mechanisms to maintain the pattern of habitat segregation displayed by these prey groups.