Physonota helianthi Randall feeds exclusively on a native sunflower, Helianthus grosseserratus Martens, in southeastern South Dakota. Helianthus grosseserratus populations are highly fragmented by agriculture and their patchy distribution presents the hypothesis that host-plant fragmentation results in genetic structuring in P. helianthi. In contrast, another cassidine beetle, Chelymorpha cassidea F., feeds on common bindweeds (Convolvulus arvensis L. and Calystegia sepium (L.) R. Br.) and therefore may not exhibit such genetic structuring. We sampled patches of host plants for P. helianthi and C. cassidea. Families were reared in the laboratory through adult emergence and assayed with starch-gel electrophoresis. Genetic variability within and among beetle families from different patches was compared for both species. For P. helianthi, 15 loci were used to evaluate allele frequencies in 30 families from five sites. For C. cassidea, we assayed 13 loci from 25 families from five sites. We determined the relative magnitude of genetic variance within a hierarchy that included families, patches, and total sample. For P. helianthi, slightly more genetic variation existed within families from the same patch than among families from different patches, but overall, variability was low (x̄ heterozygosity = 0.048). Paternity of individual egg masses was more than one in most cases. Chelymorpha cassidea was more variable (x̄ heterozygosity = 0.072) but showed less differentiation among patches. Multiple paternity was less frequent in this species than in P. helianthi. Differences in genetic structuring of these two species are likely because of their contrasting host-plant distributions and mating structure.