Using museum records, we examined sex ratios for 32 collections of eastern red bats (Lasiurus borealis) in the eastern United States. Results from logistic regression analyses indicate that female-dominated sex ratios are associated with warmer, mean high temperatures in June. Male-dominated collections, and those with approximately even sex ratios, occurred disproportionately in the Appalachian Highlands where monthly mean high temperatures were lower than in either the Central Lowlands-Interior Low Plateaus or Coastal Plain-Piedmont regions. Efforts to maximize conservation of summer roosting and foraging habitat that favors the female population segment of eastern red bats should be directed at areas where June mean high temperatures exceed 28.5 C.