The limnology of Big Bear Lake, San Bernardino County, a high mountain reservoir in the San Bernardino Mountains of southern California was studied from Nov. 1976 through Nov. 1978. Although the 1st yr of study covered a period of severe drought involving the lowest water levels in 10 yr and the lake was at or near capacity during the 2nd yr, the general limnology was similar each year. The lake typically stratifies in early spring and surface temperatures reach about 22.degree. C by mid-summer. Complete mixing occurs by Sept. Anoxia develops in the hypolimnion during stratification, increasing internal nutrient loading from sediments. The annual phytoplankton cycle peaks in the spring and late summer. Diatoms dominate in early spring, green algae briefly dominates in early summer and blue-green algae dominates from mid-summer to fall. In 1977, Anabaena and Chroococcus were the most abundant algal genera. In 1978, Anabaena dominated the early summer community, but Aphanizomenon flos-aquae dominated from late summer to fall. Algal growth appears to be limited by P from winter to spring, while N is limiting in the fall. P and N loads from tributaries draining the urbanized southeast portion of the drainage are disproportionately high. Nutrient loading rates are excessive, and well into the eutrophic range. Trophic status was similar in both years. Big Bear Lake will likely remain eutrophic because of its shallow morphology, high nutrient content and basin orientation and development.