Along the boundary between the ranges of two species of the Rana pipiens complex, Rana berlandieri and R. utricularia, in central Texas, earlier studies of male mating call (Littlejohn and Oldham, 1968) had indicated that limited hybridization is occurring in a zone of sympatry only 8 km wide. However, an analysis of electrophoretic variation in enzymes encoded by ten structural gene loci through a 100-km transect in the Austin region demonstrates that hybridization is more extensive. This study, based on 654 specimens (mostly tadpoles) collected at 13 localities in 1974 and 1975, yielded the following results: (1) Nei's genetic distance between R. berlandieri and R. utricularia in central Texas is 0.42 (based on 17 loci), which is similar to the mean value reported for nonsibling species of other vertebrates and of invertebrates. To measure genetic character, an index based on genotypes at six loci, including two with alternative alleles essentially fixed in the parental species, was constructed. (2) Individuals of mixed ancestry were not present in samples from the R. berlandieri end of the transect, and only a small proportion was found at the R. utricularia end. But from 33% to 83% of individuals collected at localities in the center of the transect were recognizable hybrids. Hybridization is not limited to formation of F1 individuals. (3) An essentially complete transition between R. berlandieri and R. utricularia occurs over a distance of 75 km, and most of the change takes place over a distance of only 36 km. (4) 'Pure' individuals of both parental species were present, in low frequency, in the center of the zone. (5) There is no evidence of introgression of genes of R. utricularia into populations of R. berlandieri, and there is little if any introgression in the other direction. The boundary between the ranges of R. berlandieri and R. utricularia corresponds with that of the Balconian and Texan biotic provinces, marked by sharp transitions in structure and composition of the vegetation and in soil-type. Although no useful information on the relative fitness of hybrid individuals is available, because the hybrid zone probably is of long-standing, the apparent lack of introgression suggests that individuals of mixed ancestry are in some way less fit, if not in the hybrid zone then in regions immediately adjacent to it. Hybridization of greater or lesser frequency is characteristic of the boundaries or narrow zones of sympatry between several other species of the Rana pipiens complex, most of which are parapatrically distributed. The occurrence in hybrid zones between pairs of differentiated populations of leopard frogs, house mice, and land snails of variant alleles that are either rare and localized or unrecorded elsewhere suggests that zones of hybridization are generally characterized by an increased frequency of mutation-like events, one of which is intracistronic recombination.