Morphologically, _Osteocephalus_ seems to be sufficiently diverse and generalized so as to represent a modern derivative of an ancestral type which might have given rise to _Phrynohyas_, _Trachycephalus_, and _Argenteohyla_. The specialized vocal sac structure in _Phrynohyas_ and _Trachycephalus_ suggests that these two genera may be rather closely allied and represent a single phyletic line from an ancestral stock similar to _Osteocephalus_. _Argenteohyla_ is quite distinct from _Phrynohyas_ and _Trachycephalus_ and apparently represents a distinct phyletic line from the ancestral stock. OCCURRENCE OF _OSTEOCEPHALUS_ IN AMAZONIAN ECUADOR All of our observations on members of this genus have been made at four localities: 1) Santa Cecilia at an elevation of 340 meters on the Rio Aguarico, a tributary of the Rio Napo, 2) Lago Agrio, 330 meters, about 14 kilometers east of Santa Cecilia, 3) Puerto Libre, 570 meters, on the Rio Aguarico just east of its formation by the confluence of the Rio Cofanes and Rio Chingua, and 4) south slope of the Cordillera del Due, above the Rio Coca, 1150 meters. _Osteocephalus leprieurii_ was found at all four localities, and _buckleyi_ was found at all but the last; _taurinus_ was found at Santa Cecilia and Lago Agrio, and _verrucigerus_ was found only in the Cordillera del Due. Our data are based on collections of 113 frogs and three lots of tadpoles, as well as observations on calling sites and young. The observations are summarized by species, as follows: _Osteocephalus buckleyi._--No breeding activity was observed. Males were found only at night in March, June, and July. One was perched on a _Heliconia_ leaf in a swamp at Puerto Libre, and two were on bushes in the forest at Santa Cecilia. A gravid female was found on a recently felled tree at Lago Agrio on the night of 12 May 1969. _Osteocephalus leprieurii._--Males were heard calling sporadically at Puerto Libre in July 1968, and at Santa Cecilia in May 1969. A small chorus was found on the night of 12 May 1969 at Lago Agrio, where the frogs were perched on branches of fallen trees over a temporary pool. The call is a soft rattling chuckle. In late April and May many gravid females and males with well-developed nuptial excrescences were obtained from trees as they were felled at Lago Agrio. The reproductive condition of the frogs indicates that they probably breed in May. One individual called nearly every night from a large tree at Puerto Libre between 4-17 July 1968. The tree was felled on the latter date, but no frog was found. Two nights later apparently the same individual called from a bromeliad at a height of about 10 m on a large bamboo adjacent to the felled tree; the frog was collected when the bamboo was cut down.