In the field, paired (in copula) female Japanese beetles (Popillia japonica Newman) tend to be larger than unpaired females. In this study, we investigated whether this size pattern could be explained by a male preference for large females, and whether larger females tended to have more and/or larger eggs than smaller females. In a laboratory study, both small and large males, when given a choice of a large and small female, tended to choose the large female. We dissected field-caught paired and unpaired females, measured their body size, and counted and measured their eggs. Larger females tended to have more and larger eggs than smaller females, indicating that males may benefit from choosing larger females due to the egg characteristics of these larger females. Paired females in the field were consistently larger than single females; paired females also had more and larger eggs, even when body width was statistically controlled. Thus, although body width of the female correlates with her fecundity, males may either use cues in addition to a female's body width to determine her immediate fecundity or fecund females may be more available for or less resistant to male mating attempts.