The Khao Chong Phran Non-hunting Area has the largest population of wrinklelipped free-tailed bats (Chaerephon plicatus) in Thailand. We examined monthly variations in sexual size dimorphism through measurements of forearm length and body mass during 2016. Bats were captured in each month at dawn and dusk. Individuals were sexed, aged, measured and marked before release. During the year, we caught 1,715 individuals. Of these, we used only adult, non-pregnant individuals (405 males and 538

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... s) for our analysis. Males had significantly larger forearm lengths and body masses than females. The body masses of both sexes peaked at the beginning and end of the rainy season, and bats captured at dawn were heavier than those netted at dusk. Seasonal fluctuations in body mass were correlated with monthly variation in rainfall; the positive correlation is likely explained by the greater insect abundance during wet months. The climate in western Thailand is expected to become warmer and drier, with likely negative effects on populations of this species due to reductions in insect food resources.