Careful study of apparently generalist phytophagous insects often reveals that they instead represent complexes of genetically differentiated host races or cryptic species. The goldenrod elliptical-gall moth, Gnorimoschema gallaesolidaginis, attacks two goldenrods in the Solidago canadensis complex: S. altissima and S. gigantea (Asteraceae). We tested for host-associated genetic differentiation in G. gallaesolidaginis via analysis of variation at 12 allozyme loci among larvae collected at six sites in Iowa, Minnesota, and Nebraska. Gnorimoschema gallaesolidaginis from each host are highly polymorphic (3.6–4.7 alleles/locus and expected heterozygosity 0.28–0.38 within site-host combinations). Although there were no fixed differences between larvae from S. altissima and S. gigantea at any site, these represent well differentiated host forms, with 11 of 12 loci showing significantly different allele frequencies between host-associated collections at one or more sites. Host plant has a larger effect on genetic structure among populations than does location (Wright's F_ST = 0.16 between host forms vs. F_ST = 0.061 and 0.026 among altissima and gigantea populations, respectively). The estimated F_ST between host forms suggests that the historical effective rate of gene flow has been low (N_em ≈ 1.3). Consistent with this historical estimate is the absence of detectable recombinant (hybrid and introgressant between host form) individuals in contemporary populations (none of 431 genotyped individuals). Upper 95% confidence limits for the frequency of recombinant individuals range from 5% to 9%. Host association is tight, but imperfect, with only one likely example of a host mismatch (a larva galling the wrong host species). Our inferences about hybridization and host association are based on new maximum-likelihood methods for estimating frequencies of genealogical classes (in this case, two parental classes, F₁ and F₂ hybrids, and backcrosses) in a population and for assigning individuals to genealogical classes. We describe these new methods in the context of their application to genetic structure in G. gallaesolidaginis. Population phenograms are consistent with the origin of the host forms (at least in the midwestern United States) via a single host shift: altissima and gigantea moth populations form distinct lineages with 100% bootstrap support. Genetic structure in Gnorimoschema is of particular interest because another gallmaking insect attacking the same pair of hosts, the tephritid fly Eurosta solidaginis, includes a pair of host races with partial reproductive isolation. Gnorimoschema gallaesolidaginis and E. solidaginis therefore represent the first reported case of parallel host-associated differentiation, that is, differentiation by evolutionarily independent insect lineages across the same pair of host plants.