Shallow-water marine molluscan faunas in Alaska and Kamchatka are the northernmost empirical evidence in the Pacific of early middle Miocene global warming, also known as the Middle Miocene Climatic Optimum (14.5 to 17 Ma). Mollusks are found in abundance in the Narrow Cape Formation on Kodiak Island (57^oN), the Bear Lake Formation on the Alaska Peninsula (56^oN) and the Kavranskaya Series in northwestern Kamchatka (62^oN). These formations contain rich, predominantly cool-water faunas, but warm-water mollusks that delineate the northern extent of the Climatic Optimum are present at a few stratigraphic intervals. This pattern of stratigraphic occurrences implies that warm surface waters of! the Climatic Optimum were not persistently present in the high-latitude North Pacific, as they were at low latitudes. Instead, relatively short-term incursions of warm surface waters from the western Pacific periodically introduced warm-water mollusks and planktonic foraminifers into this cooler region. These periodic warm-water incursions provided an opportunity for warm-temperate and subtropical taxa to migrate along the southern margin of Beringia and dwell temporarily in high latitudes. Migration patterns were asymmetrical, primarily with Asian mollusks invading waters of the northeastern Pacific. Preliminary data, based on percentages of extra-limital, warm-water taxa in several faunas, and detailed sampling for oxygen isotopes across growth increments of Anadara devincta from Kodiak Islan!d, Alaska, and A. tsudai from Rekinniki, western Kamchatka, suggest paleoseasonality ranges of 13.3 to 17.8^oC and 13.1 to 19.9^oC, respectively. Recent A. tuberculosa from the Gulf of California exhibits a higher isotope-based seasonal range of 13.9 to 28.5^oC. In contrast, Kodiak Island Dosinia ausiensis, which comes from a different stratigraphic horizon than A. devincta, indicates a wider temperature range (14.4 to 28.7^oC) than does the latter species. Modern D. dunkeri from the Gulf of California exhibits a similar maximum seasonal range of 14.5^o to 27.9^o. Compared with faunas in the modern northwestern Pacific, where shallow-sea temperatures are less affected by coastal upwelling than in the northeastern Pacific, the average temperatures of 15.5^oC (A. devincta), 16.5^oC (A. tsudai), and 21.5^oC (D. ausiensis) would occur at least 20 to 25 degrees of latitude to the south of the Miocene fossil localities. The D. ausiensis temperature range of 14.3 Centigrade degrees is comparable to that of modern subtropical mixed-layer waters, and very closely resembles the temperature ranges we obtained from recent specimens in the central Gulf of California.