 The first three specimens of "Thomasomys ucucha" were collected in 1903 at Tablón in Pichincha Province, Ecuador, by L. Söderström.It was not found again until Robert S. Voss of the American Museum of Natural History collected a total of forty-three specimens at nearby Papallacta, Napo Province, in 1978 and 1980 (the type locality is described as the valley of the Rio Papallacta).Papallacta is in a remote area that is difficult to access, and the mammal fauna of the region remains poorly known.