The systematics of the dart-poison frogs, family Dendrobatidae, experienced several taxonomic rearrangements over time (e.g., Grant et al. 2006, 2017; Brown et al. 2011). Currently, this family comprises 194 described species organized in three sub-families and 15 genera (Frost 2018). Among them, the genus Adelphobates Grant, Frost, Caldwell, Gagliardo, Haddad, Kok, Means, Noonan, Schargel, Wheeler, 2006, comprises three species, all distributed in Central and lower Amazon drainage of Peru and Brazil, and possibly in northeast of Bolivia (Grant et al. 2006; Frost 2018). Adelphobates galactonotus (Steindachner 1864) is an endemic Brazilian frog, and can be found throughout Pará, Maranhão, Mato Grosso and Tocantins states (Hoogmoed Avila-Pires 2012), related to Amazon forest formations and also in transitional areas between the Cerrado and the Amazon forest (Valdujo et al. 2012). Despite this species is classified as Least Concern (Rodrigues et al. 2010), several threats are known. First, its geographic distribution coincides with the so-called Amazonian Deforestation Arc, which comprehends the southeastern portion of the Amazonian Forest that has been rapidly converted into pasture and crop areas or being flooded due to the construction of hydroelectric power plants (Hoogmoed Avila-Pires 2012). Also, this species is present in Appendix II of CITES as a target for illegal trade, and their commercial exploitation should be controlled to avoid that this species become seriously endangered in the near future (see a case study in Paula et al. 2012). These threats are of deeper concern because despite A. galactonotus has been described since more than 150 years (Steindachner 1864), its tadpole remains unknown. Without a better understanding of the natural history of A. galactonotus, attempts of conservation strategies and population management are inefficient. In an effort to fill the knowledge gaps about this species natural history, we present a detailed description of the external morphology of the A. galactonotus tadpole.