Brazilian Merganser Mergus octosetaceus is considered to be one of the rarest and most threatened species in the Neotropical region, yet little is known about its distribution and life-history. We studied the population of Brazilian Merganser in and around Serra da Canastra National Park (SCNP) in Minas Gerais, Brazil, during 1996, with additional observations from 1997 to 2000. In this paper we report the sighting of previously undiscovered pairs and present some new behavioural data, including a description of calls performed by males and females, feeding behaviour, home ranges, parental care and population density. We also describe a previously unreported plumage of the young. A total of 39 individuals were recorded, comprising 12 adults (six pairs) and 27 young. Brood size ranged from two to four (mean 2.7), being smaller than in other Mergus spp. (e.g. Goosander Mergus merganser). Two pairs were located within the SCNP, and four on unprotected stretches of river outside. Home range was large compared with other Mergus spp., each pair using on average 9 km of river. Range size was thought to be related to the availability of suitable nesting and feeding sites. SCNP and its environs contain very little mature gallery forest with trees large enough to provide nest sites (most having been selectively logged in the past). There is also competition for nest sites with other birds and mammals. All suitable habitat surrounding the SCNP is now occupied by adult birds, leaving none for young birds. Further research is needed to determine the fate and dispersion routes of juveniles. Future conservation priorities for Brazilian Merganser in and around SCNP include the installation of nest boxes to test whether the availability of good quality nest sites limits breeding success. A captive breeding programme could be established using eggs from these nest boxes. This would help to ensure the survival of this species and to provide birds for future reintroduction programmes should they be deemed necessary. Further surveys, with subsequent establishment of reserves at key sites, are required throughout the Brazilian Merganser's range.