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The Terapa fossil vertebrate site, northcentral Sonora, Mexico, provides a rare opportunity to study the paleoenvironmental conditions present in northwestern Mexico during mid-Oxygen Isotope Stage (OIS) 3. Ostracode faunal assemblages and stable oxygen (delta O-18) and stable carbon (delta C-13) isotope values from ostracode calcite were used to reconstruct the seasonality of precipitation and vegetation cover at Terapa at 40-43 ka. The ostracode fauna was a non-analogue mix of temperate and tropical ostracode species composed of 13 species from 12 genera. The nearctic ostracodes Fabaeformiscandona caudata, Physocypria pustulosa, Cypridopsis vidua and the cosmopolitan ostracode Darwinula stevensoni dominate the assemblage. Two tropical ostracode genera, Chlamydotheca arcuata and Stenocypris sp., were present throughout the deposit and indicate that mean monthly summer temperatures were probably no more than 4 degrees C to 6 degrees C cooler than at present, based on available ecological information. Winter precipitation dominated the hydrologic cycle as evidenced by low ostracode delta O-18 values (-6 parts per thousand to -8 parts per thousand VPDB). Low ostracode delta C-13 values (-7 parts per thousand to -8 parts per thousand VPDB) suggest that local vegetation was dominated by C-3 plants. A previous tooth enamel-based paleoenvironmental reconstruction at the same site favored a summer-dominated or evaporative hydrology and abundant C-4 vegetation. The Terapa megafauna site exemplifies the need for multi-indicator paleoclimate reconstructions in desert environments where marked differences in the seasonality of precipitation and vegetation cover may occur.