The 87Sr/86Sr isotope ratios in spring waters in Mono Basin, California, are highly variable and reflect contact with different rock types. Whole rock 87Sr/86Sr ratios throughout the basin range between 0.7054 and 0.7281. Ratios in surface water samples differ greatly between runoff from the mainly granitic Sierra (0.7094--0.7097) and runoff from the volcanic regions (0.7054--0.7064). The ratio in Mono Lake water (0.7096) is similar to the ratios of streams from the Sierra Nevada. Springs close to the western shoreline have more radiogenic Sr ratios (0.7092--0.7100), while those on the east side have ratios (0.7081--0.7088) indicative of a mixture of water from the volcanic terrain and Mono Lake. Thermal springs at the south side of the lake have 87Sr/86Sr ratios of 0.7081--0.7089 and might originate in Paleozoic limestones at depth. ¿ American Geophysical Union 1995