we report numerous sightings of the sooty grassquit (tiaris fuliginosa) at four localities in the interior atlantic forest of misiones, argentina: parque provincial caá-yarí, puerto iguazú, parque nacional iguazú, and three sites at península andresito. we recorded grassquits feeding on seeds of cola de zorro (setaria sp.), and on two bamboo species: takuarusu (guadua chacoensis) and takuarembo (chusquea ramosissima). we describe the habitats in which our sightings took place, foraging behaviour, vocalizations and some aspects of breeding. we also provide data on plumage and other external features of the sooty grassquit that allow for a safe discrimination from other similar seed-eating birds in the atlantic forest. the patchy geographic distribution pattern of the sooty grassquit, its nomadism, its presence in argentina close to the end of the seeding of takuapi (merostachys claussenii) and in seeding takuarembo stands, and its abundance during the seeding of takuarusu suggest that long term survival of this species might be more dependent upon bamboo seeding than has been previously suspected.