We assessed the reproductive characteristics of the type population of Colostethus marchesianus on the banks of the Uaupés River (upper Rio Negro basin, Amazonas, Brazil). Colostethus marchesianus occupied floodplain forest areas on the river margin, and the peak of the reproductive season coincided with the peak of the rainy season. Sexual size dimorphism was not detected and adult size ranged from 15.0–17.0 mm snout–vent length. Males were territorial and employed three types of vocalization, two high-intensity advertisement calls and a lower intensity courtship call. Duration of courtship and mating behavior was less than 30 min and involved 2–3 sequential cephalic amplexi. From 1–5 egg clutches were found within each of several oviposition sites within male territories. Number of eggs in 63 clutches ranged from 10–29. Only males were observed to carry tadpoles (stage 25) to water on their backs. The mean number of transported tadpoles was similar to the mean clutch size. Additional descriptive information on reproduction and general behavior, which is highly variable among Colostethus species, is needed to confirm the specific status of other populations identified as C. marchesianus throughout the Amazonian basin.