Faunal diversity (richness) in fossil assemblages from Pliocene marine strata in California has been determined from collections from all the major Pliocene outcrops in the State. This data base consists of 750 localities, containing more than 3000 lots of fossils and almost 400 taxa. Collection, preparation, and identification were carried out by the authors in a uniform and consistent manner, thus minimizing the uncertainties inherent in interpretations based on museum collections and lists from the literature. Assemblage diversity approximates a logarithmic distribution, with many localities containing only a few taxa and few localities with many taxa. It ranges from 1 taxon (at 200 localities) to 30 taxa (at 1 locality). Diversity is determined by a combination of environmental and preservational factors. Taxa occurring alone at a locality are those that are commonly found living in single species populations (sand dollars), that have calcitic shells and so have high preservation potential (sand dollars, oysters, barnacles), or that lived in deep water where, although diversity was high, individuals were widely dispersed and so were likely to be found alone in a sample (Patinopecten, neptuneid gastropods, Macoma). At localities with 2 taxa, the taxa represent a sampling of those available rather than of interdependent pairs. Highest assemblage richness (and lowest dominance) occurs in localities from shallow open-marine settings; assemblage richness decreases (and dominance increases) with increasing water depth and, in shallow water settings, with increasing environmental stress attributed to high water energy and substrate instability and to restriction from open marine conditions.