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Descriptions of individual breeding bird biologies provide important insights to our understanding of avian life history strategies. The present study focused on the reproductive biology and nesting ecology of the Olivaceous Flatbill (Rhynchocyclus olivaceus) in the Parque Zoobotanico (-100 ha) of the Universidade Federal do Acre, Brazil, between April 2015 and March 2016. We found 91 nests, of which 14 were active and 77 were abandoned. Due to multiple nest failure incidents, we only monitored a single nest throughout the breeding period from nest construction to offspring fledging. This nest was built in 14 d, eggs were incubated for 26 d, and nestlings remained in the nest for additional 21 d. The nest was classified as closed, retort-shaped, and pensile. We deconstructed one nest that had a weight of 65 g composed of twigs (31.6% of the total weight), fibers (28.9%), leaves (14.5%), bark (9.2%), bamboo pieces (9.2%), and tendrils (6.6%). We found the nests at heights of 1-9 m above the ground, with a preference for a 2-4 m stratum, built atop 25 different plant species. The 4 species that were the most frequently used as nest substrate were Prolium unifoliolatum (n = 16 nests), Casearia sp. (n = 11), Siparuna guianensis (n = 10), and Couepia sp. (n = 9). The Olivaceous Flatbill breeds throughout the year, in both rainy and dry seasons, and it seems to be highly sensitive to human disturbance during the breeding season. The reproductive parameters recorded in the present study were consistent with those reported for other species of this genus, although most of the findings are novel for the Olivaceous Flatbill and provide important insights into its reproductive biology in the Amazon region. Received 9 September 2019. Accepted 9 December 2020. Key words: birds, Brazilian state of Acre, incubation, nest, nestlings, Rhynchocyclidae. Ecologia reprodutiva do bico-chato-grande (Rhynchocyclus olivaceus) em urn fragmento florestal amazonico no noroeste do Brasil A descricao da biologia reprodutiva de urna especie traz conhecimento importante para nossa compreensao da historia de vida das aves. Nosso objetivo foi estudar a biologia e a ecologia reprodutiva do bico-chato-grande (Rhynchocyclus olivaceus) no Parque Zoobotanico (cerca de 100 ha) da Universidade Federal do Acre, de marco de 2015 a abril de 2016. Nos encornamos 91 ninhos, dos quais 14 estavam ativos e 77 abandonados. Devido a alguns incidentes com os ninhos ativos, monitoramos apenas um unico ninho durante todo o periodo de reproducao, da construcao ao abandono pelos filhotes. Este ninho foi construido em 14 dias, os ovos foram incubados por 26 dias e os ninhegos permaneceran! no ninho por mais 21 dias. Classificamos o ninho como fechado, retorto e pendente. Dcsconstmimos um ninho de 65 g, composto por galbos (31,6% do peso total), fibras (28,9%), folhas (14,5%), cascade arvore (9,2%), partes de bambu (9,2%) e gavinhas (6,6%). Os ninhos foram construidos a alturas entre 1-9 m acima do chao, com preferencia para um estrato de 2-4 m e em 25 diferentes especies de plantas suporte. As quatro especies mais utilizadas como suporte para o ninho foram: Protium unifoliolatum (n--16), Casearia sp. (n = 11), Siparuna guianensis (n = 10) e Couepia sp. (n = 9). O bico-chato-grande se reproduz ao longo de todo o ano em ambos os periodos seco e chuvoso, e se mostrou extremamente sensivel ao manuseio humano durante o periodo reprodutivo. Alguns parametros reprodutivos revelados neste estudo foram consistentes com aqueles encontrados para outras especies do genero, porem, a maioria dos adiados e inedita para o bico-chato-grande fornecendo informacoes importantes sobre a sua biologia reprodutiva na regiao amazonica. Palavras chave: aves, estado do Acre, incubacao, ninho, ninhegos, Rhynchocyclidae., The Olivaceous Flatbill (Rhynchocyclus olivaceus) is a passerine bird of the family Rhynchocyclidae (Ohlson et al. 2013), which is endemic to the Western Hemisphere and represented by 67 species in [...]