Frogs of the genus _Ptychohyla_ are ecologically associated with mountain streams at elevations between 650 and 2200 meters; in the geographic region where these frogs occur the vegetation between those elevations consists of cloud forest or pine-oak forest. In some places the frogs have been found in a mixture of oak and semi-deciduous scrub forest. At Vista Hermosa, Oaxaca, _P. leonhardschultzei_ and _P. ignicolor_ were found in cloud forest, whereas at Agua del Obispo, Guerrero, the former species was found in pine-oak forest. _Ptychohyla schmidtorum_ is known only from cloud forest; _P. euthysanota euthysanota_ and _P. spinipollex_ generally are found in cloud forest, but in some places they live in pine-oak forest. _Ptychohyla euthysanota macrotympanum_ has been found in pine-oak forest and in a mixture of oak and semi-deciduous scrub forest. With the possible exception of the members of the _Ptychohyla schmidtorum_ group, which has been found only in cloud forest, it seems as though the type of vegetation is not the controlling factor in the ecological distribution of these frogs. _Ptychohyla_ has been found only where there are clear, cascading streams overhung by vegetation, on which adults and young perch at night, or even by day. The presence of these streams, in which the tadpoles live, seems to be an important factor in the distribution of _Ptychohyla_. As has been shown previously, the tadpoles of _Ptychohyla_ are adapted for existence in torrential streams, where the water is cool, and the amount of oxygen is high. Clearly these tadpoles are unsuited for life in ponds or sluggish streams in the lowlands, where the temperature of the water is high, a layer of silt on the bottom is deep, and the amount of oxygen is low. The tadpoles cling to rocks on the bottom of the streams; there they move slowly across the rocks, apparently feeding on the thin covering of algae. Tadpoles were not observed on rocks having a thick covering of algae or moss. The tadpoles were observed to swim against the current in torrential streams, in which no fishes were found. Therefore, it seems as though the presence of the stream-habitat for the tadpoles is a significant factor in the ecological distribution of the species of _Ptychohyla_. Interspecific Competition