At localities where two species of _Ptychohyla_ occur sympatrically (_P. ignicolor_ and _P. leonhardschultzei_ at Vista Hermosa, Oaxaca, and _P. euthysanota euthysanota_ and _P. schmidtorum schmidtorum_ at Finca La Paz, Depto. San Marcos, Guatemala) effort was made to determine what, if any, ecological interspecific relationships existed. Although adults of the sympatric species were found on adjacent leaves or branches of bushes overhanging the streams at both localities, segregation at the time of breeding seems to be maintained by the notably different breeding calls in sympatric species (see discussion of breeding calls). Thus, as has been shown by Blair (1956), Fouquette (1960), and others working on a variety of pond-breeding frogs and toads, the breeding call in _Ptychohyla_ acts as an important reproductive isolating mechanism. At no locality were _Ptychohyla_ and associated species of hylids found so abundantly as were species of pond-breeding hylids in the lowlands. Apparently reproductive activity is not concentrated in a short breeding season, and it is highly doubtful if the populations of these frogs are as large as those of the lowland pond-breeders. The continual humid conditions and abundance of insect food throughout the year in the cloud forest are perhaps indicative of little interspecific competition among adults of _Ptychohyla_ and other sympatric hylids. At Finca La Paz, Guatemala, tadpoles of two species of _Ptychohyla_ were ecologically segregated. The tadpoles of _P. euthysanota euthysanota_ were found in riffles in the streams, whereas those of _P. schmidtorum schmidtorum_ were found in slower water, chiefly in small pools in the streams. At Vista Hermosa, Oaxaca, tadpoles of _P. leonhardschultzei_ were found in riffles, and tadpoles of the sympatric _P. ignicolor_ were found in a small pool in a stream. Similar ecological relationships were observed for several species of Costa Rican hylids. Throughout the range of _Ptychohyla_ east of the Isthmus of Tehuantepec, members of the genus occur with species of _Plectrohyla_, all of which are larger than _Ptychohyla_, and all of which have tadpoles that live in torrential streams. Tadpoles of _Ptychohyla spinipollex_ have been found in streams inhabited by the tadpoles of _Plectrohyla guatemalensis_ and _P. quecchi_; tadpoles of _Ptychohyla euthysanota euthysanota_ and _P. schmidtorum schmidtorum_ were found in streams inhabited by tadpoles of _Plectrohyla guatemalensis_, _P. matudai_, and _P. sagorum_. In some streams great numbers of tadpoles occur. The habitat is rather restricted, and the food supply is limited. Consequently, interspecific competition among the various species of hylids whose tadpoles live in the torrential streams probably is highest during the larval stage. Unfortunately, this aspect of salientian population ecology has received no intensive study. Reproduction and Development