Male insects often compete for females through displays or combat. Where males fight over females, larger males generally have reproductive advantages over smaller males. In contrast, female insects often discriminate against potential mates based on body size. Female discriminating against males can occur prior to mating (pre-copulatory), during mating (copulatory), and following mating (post-copulatory). Northern corn rootworms (NCR) are sexually dimorphic with respect to head capsule width, which correlates with pupal and adult body weight. We report on aspects of NCR reproductive biology in relation to male and female size. We used pupal weight as initial size indicators, and paired combinations of large and small males with large and small females in 60 x 15 mm petri dishes. Large and small individuals were > and < 1 SD from the mean. Average sized males and females were also paired for comparisons, which were also within 1 SD of the mean for each sex. To insure virginity, all individuals were obtained as pupae and reared to adults. Mating pairs were used only once. We videotaped all unsuccessful mating pairs for at least two hours and successful mating pairs until copulation was completed. Females were housed individually with ample food, water, and oviposition dishes and placed in growth chambers. Oviposition dishes were changed weekly until the females died. We report the data with respect to mating success and fecundity based on male and female size. We discuss our findings with respect to the evolution of resistance to transgenic corn targeting corn rootworms.