“Blue Wall”, in San Andrés Island (Colombia), is an outer reef slope with an inclination that increases with depth to nearly 90°. Such a steep gradient facilitates the identification of patterns of variation in species abundance and diversity along depth. To document such patterns in fishes associated with this habitat, 30 visual censuses were conducted along 30 x 2 m transects, located at 5 m intervals between 5-30 m depth, in which the abundance and diversity of species was estimated. A total of 2916 individuals belonging to 46 species from 21 families were recorded within transects. Ten additional species were observed outside of transects for a total richness of 56 species. The dominant species were Canthigaster rostrata, Chromis cyanea, Stegastes partitus, Thalassoma bifasciatum and Clepticus parrae, which together with four other species accounted for 92% of the total abundance. Several of these dominant species are planktivorous and abundant throughout the Caribbean and had broad ranges of vertical distribution with abundance peaks in different parts of the depth gradient. In general, there was a positive correlation between average abundance and breadth of depth distribution. Non-metric Multi-Dimensional Scaling analysis revealed distinct assemblages at each depth, which also differed significantly, but weakly, in their diversity and dominance, but not in their species richness and evenness. Shallow assemblages were clearly distinctive from deep ones, with greater differentiation between shallower than between deeper assemblages. We conclude that the fish assemblage at “Blue Wall” exhibits a structural order that can be explained, in part, by depth and that reef walls offer an excellent opportunity to examine processes responsible for patterns of community structure in coral reef fishes.