In insects, larger males generally have a reproductive advantage over smaller males when competing for mating partners. We examined male reproductive competition together with precopulation and copulation durations, female longevity, and fecundity in the northern corn rootworm in relation to the body size of males and females that were combined for mating. Longevity and fecundity were determined for individually caged, mated females. Of the females in 108 combinations of two males and one female, 35 chose not to mate. Of the females in the 73 combinations that resulted in copulation, 38 were small and 35 were large. The proportions of large and small males that mated did not vary significantly with female size, but large males were more than twice as likely as small males to mate. The precopulation duration did not vary with either male or female size, and no interaction occurred between male and female size for either the precopulation or copulation duration. However, both male and female size affected the duration of copulation, with small males copulating longer than large males and large females copulating longer than small females.Nofemale longevity or egg number differences occurredamongthe body size categories of the mating pairs. The implications of the results for insect resistance management are discussed, considering that the evolution of resistance to certain management strategies, such as resistance to the use of insecticides or of Bt maize, may be accompanied by changes in body size.