"Rapoport's rule," which has gained wide acceptance as a potential explanation for latitudinal and other diversity gradients, holds that mean latitudinal range of species decreases toward the equator. We analyzed latitudinal ranges of 2838 eastern Pacific marine molluscan species, a subset of which figured in the original formulation of Rapoport's rule, and failed to find the predicted trends. Instead, species diversity gradients and range magnitudes appear to vary independently, with the spatial distribution of major oceanographic barriers exerting a strong influence on latitudinal ranges. Biogeographic structure should, therefore, be an important element in the assessment of diversity patterns.