Coleomegilla maculata, an abundant, widely distributed, North American polyphagous beetle, is a predator of soft-bodied insects including aphids. Gene diversity and breeding structure of 12 C. maculata populations was investigated by using polyacrylamide gel electrophoresis. Thirty-five of 44 putative loci were polymorphic (79.5%). Genic heterozygosity was estimated at 28 polymorphic loci. Average heterozygosity among the 37 scorable loci was 18.3 ± 3.5% and was 24.2 ± 5.0% among only polymorphic loci. The average number of alleles was 4.00 ± 3.01 and the effective number of alleles was 1.61 ± 1.65 among the 37 loci. No null alleles were recognized. Forty-five putative alleles segregating at 7 presumptive loci were used to study breeding structure. A significant deficiency of heterozygotes was detected within populations; this deficiency was a result our failure to identify heterozygotes for alleles of similar mobilities. No significant differentiation was found among populations within fields, among populations within states, or among states. Gene frequencies did not significantly differ between beetles sampled in autumn nor among their progeny the next spring. There were no significant departures from random mating among populations.