Pliocene sediments of the Kettleman Hills, California, were deposited in a marine embayment that occupied part of the San Joaquin Valley. Faunal and sedimentologic data from the upper Pliocene Pecten zone indicate a decreasing salinity trend south and southwestward across the Kettleman Hills. In general, the least diverse faunas and coarsest sediment are in the west-central part of the area, near an apparent small delta and are relatively distant from the entrance of the embayment. Salinity and substrate are probably the main factors controlling the composition of the eight fossil communities recognized by Q-mode cluster analysis. Oxygen and carbon isotopic age determinations were made on specimens of Pecten, Aequipecten, Ostrea, and the fresh-water clam, Anodonta, in order to determine paleosalinities. Assuming that Anodonta grew in water that was representative of the fresh water that flowed into the embayment, that the shells with the heaviest isotopic composition grew in normal marine water, and that the temperature was constant throughout the area at that time, the salinity at which the shells grew can be calculated from their oxygen isotopic composition. The salinities calculated by this procedure range from normal marine (35 ‰) in the north and east parts of the Kettleman Hills to brackish (about 10 to 15 ‰) in the west-central part. These results are in good agreement with those suggested by faunal composition and diversity. A similar but more limited study of the stratigraphically higher upper Mya zone indicates that the salinity during this time was reduced but did not vary in a systematic pattern as during Pecten zone time. Comparison of fossil and modern communities provides the means for detailed paleoenvironmental reconstruction by establishing analogous modern settings. Environmental interpretation, based on isotopic analysis, is similar to interpretations based on faunal analyses but, in addition, provides more quantitative estimates of the paleotemperature and salinity.