Specimens examined.--OAXACA: Cerro San Felipe, UIMNH 25050. "Mexico," MNHN 6331. RELATIONSHIPS The evolutionary trend in the members of the _Hyla bistincta_ group is towards aquatic habits. _Hyla bistincta_, the least specialized species in the group, has relatively short fingers, webbing between the fingers, a truncate, high snout, and relatively large subarticular and supernumerary tubercles. _Hyla charadricola_ resembles _bistincta_ in having relatively short fingers, a slight amount of webbing, and a truncate snout. Apparently these two species are more closely related to one another than either is to the other species in the group. _Hyla robertsorum_, _pachyderma_, and _crassa_ are the most aquatic members of the group. These species are closely related, possibly conspecific. All have round, sloping snouts, robust forearms, long, unwebbed fingers, and large webbed feet. Both _H. pachyderma_ and _H. crassa_ seem to be advanced beyond _H. robertsorum_. If small nuptial spines, moderately webbed feet, and absence of a well-defined thoracic fold are considered to be less advanced than large nuptial spines and a strong thoracic fold, as in _H. pachyderma_, or fully webbed feet, as in _H. crassa_, then _H. robertsorum_ must be considered to be less advanced than _H. pachyderma_ or _H. crassa_. Members of the _Hyla bistincta_ group inhabit mountain streams. The frogs can be found along these streams throughout the year. Since in most stream-breeding hylids there is no migration to breeding sites, the breeding call does not function to attract females to the breeding site. Apparently voices are lacking in all members of the _Hyla bistincta_ group, except in _Hyla bistincta_. The presence of vocal slits and the ability to call further indicate that _Hyla bistincta_ is the primitive member of this group. Members of the _Hyla bistincta_ group and the species of _Plectrohyla_ closely resemble each other in osteology and body form of the adults and in structure of the tadpoles. This resemblance suggests a close relationship between the two groups. _Plectrohyla_ apparently evolved from an ancestral stock resembling the extant _Hyla bistincta_. Probably this stock gave rise independently to _Plectrohyla_ and to the _Hyla robertsorum-pachyderma-crassa_ complex. In the former the voice was retained, and a projecting prepollex spine developed, whereas in the latter the voice was lost, and the prepollex spine did not project. _Plectrohyla_ lives in mountain streams in the Chiapan-Guatemalan highlands; the _Hyla robertsorum-pachyderma-crassa_ complex inhabits similar environments in the Sierra Madre Oriental in México. _Hyla charadricola_ also lives in the Sierra Madre Oriental, whereas _Hyla bistincta_ is widespread in the mountains of México southeastward to the Isthmus of Tehuantepec. LITERATURE CITED BARBOUR, T. 1927. _Cauphias_ rediscovered. Copeia, no. 165:96-98, December 23. BROCCHI, P.