Allelic frequencies at certain genetic loci, estimated by electrophoresis of enzymes and other proteins, in Bufo cognatus populations sympatric with B. speciosus are more similar to the allelic frequencies of B. speciosus than are corresponding frequencies in B. cognatus populations that are allopatric to B. speciosus. There is no corresponding trend in B. speciosus. There is an opposite trend in morphological data, with the exception of one B. cognatus population. These B. cognatus, from El Paso Co., Texas, also show stronger similarities to B. speciosus in allelic frequencies than do other B. cognatus populations. Assuming that the proteins are less susceptible to direct environmental modification than is morphology, and are, thus, better indicators of genetic differences, the data presented here suggest gene flow from B. speciosus to B. cognatus. No evidence of reciprocal flow from B. cognatus to B. speciosus is apparent. INTRODUCTION In contrast to many groups of animals, natural hybridization between distinct species of Bufo is fairly frequent (Mayr, 1963). Numerous natural interspecific hybrids have been reported (Ballinger, 1966; A. P. Blair, 1941, 1942; W. F. Blair, 1957; Brown, 1967; Henrich, 1968; McCoy et al., 1967; Thornton, 1955; Volpe, 1952). In addition, many combinations of species produce viable hybrids in laboratory crosses (W. F. Blair, 1959, 1961). But what is the extent to which genetic material of one species is incorporated into the gene pool of another species or the extent of introgressive hybridization, to use a term coined by Anderson and Hubricht (1938) ? The extent of introgression, or lack of it, has usually been inferred from comparisons of morphological characteristics of species pairs (Anderson, 1949). Although this method has been useful in detecting hybridization and arriving at estimates of its effect, it would only rarely provide information about effects at the level of single loci, due to the polygenic determination of most morphological characters. This difficulty has been partly removed by gel electrophoresis and histochemical staining of proteins, allowing the study of characteristics controlled by only one or a few loci. These methods have already been useful in studying introgression in snails (Manwell and Baker, 1968), fish (Hagen, 1967), toads (Guttman, 1967), turtles (Crenshaw, 1965) and lizards (Gorman and Dessauer, 1966). 1 Part of a dissertation submitted to the Faculty of the Graduate School of the University of Texas at Austin in partial fulfillment of the requirements for the Ph.D. 2 Present address: Department of Biological Sciences, Louisiana State University in New Orleans 70122.