Little is known about density and structure of black coral populations of the continental Pacific coasts of Central and South America. Species diversity and ecology of the antipatharian fauna of Machalilla National Park (Province of Manabí, Ecuador) were surveyed using scuba, and two species, Myriopathes panamensis and Antipathes galapagensis, were identified. New information on the two species and their associated fauna was obtained through both underwater observations and laboratory analyses. Specific associations with stalked barnacles and parasitic zoanthids are described. An underwater visual census indicated that the black coral assemblage had a maximal density between depths of 15 and 30 m. Myriopathes panamensis commonly occurred below 20 m depth, and A. galapagensis was mainly recorded from deeper than 25 m depth. Surveyed sites were characterized by sparse rocks mixed with sandy patches, and occurrence of black corals was mainly related to availability of rocky substrate. With an average density of 0.5 colonies m-2, the shallow black coral community of Machalilla National Park is one of the densest in the world. Data from this study represent a clear baseline for monitoring of population dynamics of benthic organisms in an area subjected to periodic El Niño and La Niña events, which may greatly affect composition and abundance of the marine communities.