This study of genetic relationships and reproductive isolation in southeastern hylid frogs of the genera Pseudacris and Hvla places particular emphasis on experimental evidence obtained from interspecific crosses. It investigates the degree of correlation between hybrid success and phylogenetic relationship and the potentialities for interspecific gene exchange. Species of Pseudacris primarily studied were P. triseriata, P. brachyphana, P. nigrita, P. brimleyi, and P. ornata, although pertinent information relating to P. streckeri and P. clarki was also included. The species of Hyla studied were H. crucifer, H. cinerea, H. gratiosa. H. versicolor, H. avivoca, H. femoralis, and H. squirella. The relationship between hybrid success and phylogeny is complex. Levels of normal development in the F1 from crosses between forms assigned to the same species group were usually high, but exceptions were encountered. Conversely, development in crosses between distantly related species was usually low or absent, but some crosses yielded good results. Similarity between certain species in patterns of crossability with other species and in kinds of embryonic abnormalities in the F1 did seem to confirm presumed relationship in some cases. A more consistent picture was obtained with regard to hybrid sexual development and fertility. Some abnormal development of the primary sexual structures was found in most viable hybrids between members of different species groups, and a shift in the sex ratio in the direction of males was common. Hybrids were usually highly sterile, and although limited gametogenesis was demonstrated in isolated instances, the gametes appeared to be poorly viable and the possibility for successful backcrossing remote. On the other hand, normal sexual development and at least some (often very high) fertility was characteristic of hybrids between species within the same species group. The only exception would seem to be the hybrid between the 2 allopatrically distributed species of the P. ornata group. The potentiality for natural hybridization and introgression between the various species was evaluated on the basis of development of the various types of reproductive isolating mechanisms. It was concluded that a potentiality for interspecific gene exchange with at least one sympatric species exists for 5 of the 7 species of Pseudacris and 5 of the seven species of Hyla considered. This conclusion is substantiated by known instances of natural hybridization in 5 species combinations and one documented case of introgressive hybridization.