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Changes in ostracode species assemblage suggest presence of oligohaline and mesohaline waters in the Santiaguillo basin of subtropical Mexico over the last ca.27 cal kyr BP. Paleoecological reconstruction is based on the distribution of Limnocythere bradburyi Forester, Eucandona cf. patzcuaro Tressler, Cypridopsis vidua Brady and Limnocythere ceriotuberosa Delorme. Source of organic productivity is inferred from C/N ratios. The basin hosted shallow and oligohaline water during the late glacial (ca.27-19 cal kyr BP) and lacustrine phytoplankton contributed to the organic productivity However, the water body of Last Glacial Maximum (LGM) was relatively warmer (>13 degrees C) and more diluted compared to the pre-LGM. Increased salinity and reduced lacustrine productivity at ca.19 cal kyr BP marked a shift in the paleoecological conditions. Water body was mesohaline during ca.9.5-4 cal kyr BP and periodic desiccation of the basin possibly caused the absence of ostracode valves in sediments of the last ca.3.5 cal kyr BP. Comparison with other proxy-records from the region indicates that the hydrological variations in different parts of subtropical north and northwestern Mexico were not synchronous during the late Pleistocene and Holocene.