Female insects may choose to mate with males providing the largest nuptial gift via the spermatophore, which may correlate with fitness related characters such as body size. Here, we examined spermatophore size of northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), in relation to body size of males and females and in relation to pairing duration. Virgin males and females of known ages were weighed before and after pairing. Pairs were noted as copulating or not copulating. Copulating males lost significantly more weight than did noncopulating males, whereas copulating females gained significantly more weight than did noncopulating females. For copulating pairs, male and female weight before pairing correlated positively. Spermatophore weight correlated positively with male weight before pairing and was estimated to be 0.37 or 0.38 mg, depending on whether weight change data from males or females were used for estimation. Spermatophore mass accounted for ≈4.4% of male body weight. An effect of pairing duration on spermatophore weight was demonstrable only when weight change data from females were used. Implications of the results for management of rootworm resistance to genetically modified maize are discussed.