Egg fertility, embryo viability, and fecundity were measured, using experimental hybridization, to produce a profile of reproductive compatibility across a hybrid zone between Hyalophora euryalus (Boisduval, 1855) and H. columbia gloveri (Strecker, 1872) (Lepidoptera: Saturniidae) in the Sierra Nevada mountains of California. Three aspects of the genetic structure of the hybrid zone are discussed: 1) hybrid zone females have normal fecundity, in contrast to typically barren laboratory-reared F₁ hybrid females from crosses between widely allopatric parental taxa; 2) hybrid zone females from defined sites along the transect, in comparison to allopatric species matings, showed no reduction in fertility or viability in matings with males from the same site; and 3) a “hybrid sink” was discovered, the moths from which were the least compatible among various inter-site crosses. Topography affects the genetic structure of the hybrid zone, especially by reducing gene flow in relation to rain shadows and associated ecological features. Within the hybrid zone, natural selection has apparently modified postzygotic isolation by favoring compatible genotypes regulating öogenesis. The long-term structure and stability of the hybrid zone is discussed in terms of ecological heterogeneity in complex montane landscapes. The suitability of the “tension zone” model, which depicts an equilibrium between the effects of selection balanced against dispersal, is discussed for the Hyalophora hybrid zone.