Diabrotica spp. are serious pests of maize, Zea mays L., and have evolved resistance to cultural and chemical management practices. Transgenic maize producing a rootworm-toxic protein derived from the bacterium Bacillus thuringiensis Berliner provides a new approach to Diabrotica spp. management. Its use requires a refuge of nontransgenic maize to produce susceptible beetles able to mate with any resistant individuals developing on the transgenic variety to slow the evolution of resistance. Such evolution may simultaneously affect fitness related traits such as longevity, fecundity, and body size. We examined the mating behavior and reproductive biology of Diabrotica barberi Smith & Lawrence in relation to male and female body size. Large and small males were reciprocally paired to large and small females for comparison with average sized beetles. Neither the proportion of pairs that copulated nor the precopulation duration varied significantly with size category; however, the duration of copulation was shortest for large males crossed with small females and longest for the reciprocal cross. Large females lived the longest regardless of mating partner. Although small females mated to small males tended to lay the fewest eggs, size category did not significantly affect egg numbers per week of life during a 12-wk oviposition period. Egg production peaked during the first 4 to 5 wk and then steadily declined. These results suggest that resistance to transgenic maize, if associated with small body size, could provide a reproductive disadvantage for resistant females but not males that could prolong efficacy of the transgenic maize.