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Known nocturnal behaviors of bees in the superfamily Apoidea, including the genus Bombus, were almost exclusively of females. Here we report observations of active free-ranging male Bombus at night in the plant nursery of the Fushan Research Center, Taiwan, in April 2022. Nectar feeding by males at inflorescences was confirmed by tongue-licking in the absence of pollen collecting. The numbers of active female and male bumblebees during the daytime were close to equal. In contrast, only males were found to be active in the night. Our observations suggest that such nocturnal activity is facultative. This finding not only provides a rare case of nocturnal activity in free-ranging Bombus, but also demonstrates that such behaviors can vary between the sexes.