Tadpoles of two closely related Amazonian species of Colostethus are described. Colostethus marchesianus was described by Melin in 1941 from Missão Taracuá, Rio Uaupés, Amazonas, Brazil, and has been depicted as widely occurring throughout the Amazon region; Melin did not describe the tadpoles of this species. Subsequent descriptions of the tadpole of C. marchesianus have been based on specimens from Ecuador, Peru, and from north of the Amazon River in Amazonas, Brazil. However, variation among populations referred to as C. marchesianus has indicated that this taxon may actually be a complex of undescribed species. We revisited the type locality to obtain specimens of C. marchesianus with the objective of redescribing the adult and describing the tadpole for the first time. We also obtained tadpoles of a recently described species, Colostethus caeruleodactylus, that occurs south of the Amazon River in Amazonas, Brazil, and is similar to C. marchesianus. Tadpoles of the two species can be separated from each other based on the reduced number of tooth rows and other features of the oral disc. We suggest that size, arrangement, and number of papillae may be a synapomorphy uniting members of a clade consisting of C. marchesianus and C. caeruleodactylus and possibly closely related species whose tadpoles are unknown at present.