The geology, physical conditions, size, and position within the Caribbean may all play a role in shaping the fauna of islands in the Mona Passage. This paper describes the ichthyofauna of Mona, Monito,and Desecheo by habitat, compares it to surrounding island faunas, and discusses the zoogeography of its shore fishes. We used ichthyocides and diver visual survey during several expeditions to collect and identify the shore fishes from the islands. Islands in the Mona Passage have a diverse though perhaps impoverished marine ichthyofauna consisting of 261 known species, compared to nearby Puerto Rico that has more than twice as many species. The narrow shelf area of these islands compresses reef zones that are typically distinct and spread over a much broader shelf. Species typical of the same reef zones in Puerto Rico also occur at Mona, but shelf-edge species can be found nearshore. The limited habitat diversity, including restricted mangrove and seagrass areas, may have an effect on the marine fishes by excluding some species and reducing the abundance of others. These islands fall within a major faunal break between Hispaniola and Puerto Rico. This biogeographical barrier is an imperfect filter for marine fishes as some are found east of the Mona Passage, but are absent from the Puerto Rican Plateau and Lesser Antilles. While several sampling methods were used, ichthyocide collections accounted for the largest number of species. The true diversity of the ichthyofauna would be hidden without the use of ichthyocides to sample the cryptic fish assemblage.College of Arts and Sciences University of Puerto Rico, Mayagüez