In contrast to most odon. females which mate repeatedly, females of I. verticalis were found to be effectively monogamous, remating only if sperm loads were low enough to cause substantial sterility of egg clutches. A female’s colour phase corresponded to her sexual maturation. Virgin and non-virgin females were non-randomly distributed in the field and responded differently to male approaches. Both the penis structure and the behaviour of males during copula suggested that males could displace sperm if given the chance to do so. However, evidence of sperm removal using the volume of sperm storage organs of females interrupted in copula was equivocal. I. verticalis females are apparently monogamous because they do not benefit by mating repeatedly and cannot be forced to mate. As predicted for a monoandrous mating system where the potential opportunity for sexual selection on males is relatively low, males showed no overt forms of male-male competition, and were repelled by females giving a wing-flutter display.