Engystomops petersi (common name: Peters' dwarf frog) is a species of frog in the family Leptodactylidae. It is found in Amazonian Colombia, Ecuador, and Peru. It is morphologically similar to its sibling species, Engystomops freibergi, and for a period the latter was considered to be a junior synonym of Engystomops petersi. Taxonomy and classification of this species is constantly changing due to the continual evolution of behavioral isolation and rapid speciation in the region. There are also records from the Guianas that have not yet been allocated to either species. Divergence of these two species seems to have been driven by behavioural isolation related to male call characteristics more than geographic isolation. Taxonomy The genus Engystomops is part of the family Leptodactylid. Previously, the genus Engystomops was grouped in with other Amazonian genera like Eupemphix and Physalaemus. These names are routinely exchanged making it hard to characterize differences among different species. Previously, Eupemphix paraensis, Eupemphix schereri, and Engystomops petersi were considered to be distinct species, however morphological analyses of these frogs revealed that they were conspecifics and synonymous species. Description Engystomops petersi are relatively small frogs. Males measure in snout–vent length and females . Dorsal colouration is variable. Skin on the dorsum is warty, bearing small tubercles with scattered larger tubercles. This species lacks maxillary and premaxillary teeth. E. petersi also has terminal phalanges that are T-shaped and two salivary glands known as parotoid glands that are generally visible. The hands and feet of E. petersi are characterized by bulbous fingers in which the first finger is greater in length compared to the second finger. Their build varies from slender to stocky. They are also distinct from other similar frogs in that some females have concealed eardrums or tympani and males have prominent tympani. Larval Morphology The larval morphology of E. petersi has been thoroughly studied and compared to other members of the Engystomops genus. E. petersi larvae are generally perceived to be the largest in size of all Engystomops larvae. Its larval structure is mainly composed of the body, snout, oral disc, vent tube, tail, dorsal fin, ventral fin, and other organs. The length of the larva usually is around 22.7 mm. Its body length comprises a little less than half of its total length. It has a broad snout that is wide and round with nostrils that are slightly discolored on the outer rim. Its oral disc is