The 957-ha forest at the Estação Biológica de Caratinga/Reserva Particular do Patrimônio Natural-Felìciano Miguel Abdala, in Minas Gerais, Brazil, supports one of the largest known populations of the critically endangered northern muriqui (Brachyteles hypoxanthus). We combine long-term data on one group that has been monitored since 1982 with new data obtained on the other three groups since 2002 and 2003 to describe the demographic structure of this population, evaluate its potential for future growth, and predict how dispersal and competitive regimes may change in response to current demographic conditions. As of January 2005, the 226 individuals in the population were divided into four mixed-sex groups with 37-77 members, and an all-male unit whose eight males maintained transient associations with two of the mixed-sex groups. Although 51.77% of the population was female, the sex ratio among adults and subadults was female-biased (0.75), while that among immatures was male-biased (1.47). Consistent with expectations from mean interbirth intervals, 64.18% of adult females gave birth in 2003 and 2004. However, by January 2005, only 52.31% of adult females were still carrying infants <24 months of age due to unusually high infant mortality. First-year survivorship among the 25 infants born in the population in 2003 was only 76%, considerably lower than previously documented in the longest-studied group. High female fecundity is indicative of a healthy population, but the current male bias in births will result in a decline in the population growth rate within two decades, and may increase levels of male reproductive competition and alter dispersal patterns. (c) 2005 Wiley-Liss, Inc.