Summary.The interstitial cells of the testis ofPteropus geddieiwere found to be most numerous and their volume to be greatest when copulation occurs and for the three months preceding this. It is therefore supposed that the greatest quantity of male hormone is elaborated at this time, October‐February, when the bat is social and sexual.From March to June there is moderate glandular activity, the behaviour of the bat being social and asexual.In July and August interstitial activity is at its minimum, and the bat is solitary and asexual.There is a significant seasonal variation in the interstitial tissue of these bats, living in a remarkably constant tropical climate; although the variation is less pronounced than that found in British Wood Mice.It is suggested that the male hormone may to some extent control the purely social behaviour ofPteropu