The Pleistocene paleoenvironments of coastal San Diego County, California, clearly have been affected by eustatic changes in sea level induced by glaciation events in the Northern Hemisphere. Two distinct marine abrasion platforms were incised into the local coastline during late Pleistocene time. As a result, two marine terraces, which are the modern topographic expressions of ancient abrasion platforms, are recognized along the San Diego County coast--the Nestor terrace, formed 120,000 ± 10,000 yr BP during a stillstand of the sea 6 ± 4 m above the present sea level, and the Bird Rock terrace, formed 80,000 yr BP during a stillstand 14 ± 2 m lower than present sea level. Local tectonic upwarping has raised the terraces to their present elevation. Pleistocene marine invertebrate faunas are found preserved in fossiliferous deposits on marine terraces and in other sedimentary exposures in San Diego County. Data from the sedimentology and paleontology of each specific fossil locality allowed paleoenvironmental reconstructions at individual sites. The paleoecology of the species present and the extralimital species found within the fossil faunas are prime indicators of the paleoenvironmental setting, and climatic and water temperature conditions, respectively. Ecologically mixed faunas at numerous localities suggest the proximity of varied microhabitats. Key species for particular molluscan associations can be recognized in Pleistocene faunas, e. g., Donax gouldii and Tivela stultorum indicating open sandy beaches, Tagelus californianus and Nassarius tegula representing sheltered muddy lagoons, and Littorina planaxis and Mytilus californianus indicating exposed rocky shores. Other taxa may be restricted to specific depth ranges, substrate, salinity tolerances, and temperatures. Paleoenvironmental reconstructions of forty-one Pleistocene fossil localities provided information on San Diego County's physical setting and climatic conditions. Amino acid dates on twenty-nine localities, together with the climatic and water temperature data derived from extralimital species in the faunas, provided a chronologic paleoenvironmental overview of San Diego County in Pleistocene time during episodes of high sea level stands. Two intervals of early Pleistocene time indicate the possibility of warmer climatic conditions than exist today. The middle Pleistocene time interval (500,000-600,000 yr BP) probably had a climate with water temperatures warner than today, while the second middle Pleistocene interval (250,000-300,000 yr BP) exhibited a general paleoclimate very similar to today, with warm-water species restricted to the protected embayments and sheltered lagoons. The late Pleistocene time (120,000 yr BP) was concurrent with the formation of the Nestor terrace and probably exhibited warmer climatic and water temperature conditions than are present at San Diego's latitude today. Both a lesser volume of glacier ice during this stillstand 6 m above present sea level and the notable presence of southern extralimital species substantiate these postulated warmer conditions. But at 80,000 yr BP during a stillstand of the sea 14 m lower than present sea level recorded by the Bird Rock terrace, cooler climatic and water temperature conditions existed. The presence of northern extralimital species and the probability of more glacial ice support this interpretation.