Concern regarding the ecological impact of rapid global warming has encouraged research on climate-induced changes in biological systems. Critical problems, still poorly understood, are the potential for rapid adaptive responses and their genetic costs to populations. The O chromosomal polymorphisms of Drosophila subobscura have been monitored at a southern Palearctic locality experiencing sustained climate warming since the mid-1970s. Observations suggest that the population is rapidly evolving in response to the new environmental conditions, and has lost a significant amount of chromosomal diversity (18.3% in 16 years). These findings are consistent with results from another population of D. subobscura, which is also undergoing climate warming, and are in accord with what would be expected from latitudinal and seasonal patterns of the various inversions. In addition, data on the O chromosomal polymorphisms from other localities throughout t he range of this species suggest that other populations vary similarly.