Diabroticite corn rootworms are significant maize, Zea mays L., pests that have evolved resistance to cultural and chemical management methods. Transgenic maize synthesizing a rootwormtoxic protein originating from the bacterium Bacillus thuringiensis Berliner is an important new management technology. Its use requires a refuge of nontransgenic maize to delay evolution of resistance by generating susceptible beetles capable of copulating with any resistant individuals surviving on the transgenic crop. The evolution of such resistance may concurrently influence a variety of fitness-related characters. Here, we examined multiple mating by female Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae) in relation to the following fitness-related traits: female longevity, fecundity, and body size of males and females at the first copulation. For the first copulation, large and small males were reciprocally paired to large and small females for comparison with average-sized beetles. Once-mated females were then maintained under conditions conducive to oviposition and paired with averaged-sized males at weekly intervals until death. Although female age at second copulation varied significantly with size category, female age at first copulation, longevity and fecundity did not. Females that mated multiple times lived longer and laid more eggs than did those that mated once. The results suggest that resistance management models may need to consider those females deriving fitness benefits from a second mating.