The Critically Endangered tamaraw Bubalus mindorensis is endemic to Mindoro Island, Philippines, and little is known of its ecology. During 2006-2011 we used community-based monitoring to examine the population status and ecology of tamaraw in the species' core habitat of Mount Iglit-Baco National Park. Each year, for 5 consecutive days at the end of the dry season, trained local volunteers and rangers or project staff were allocated to 18 vantage points in the study area (c. 160 km(2)). Tamaraw were categorized as adult (> 5 years), juvenile (2-5 years) or calf (< 2 years), and sexed when possible. During the study period the population was 239-314 (mean 271), with no significant fluctuations in age structure (percentage of adults, juveniles and calves: 57.8, 21.0 and 21.3%) or estimated adult female reproductive rates (29.1%). In adults, but not in juveniles, the sex ratio was biased towards females (1 : 1.86, P < 0.01). Bulls were often solitary (32.2% of sightings), whereas the majority of cows (94.7%) formed small groups of 2-12 individuals of different ages, with or without bulls (53.4 and 46.6%, respectively). These results demonstrate that the population remained relatively stable, maintaining a constant age structure and reproductive rate, and that long-term community-based monitoring was effective for quantitative characterization of the tamaraw's social behaviour, which is critical for conservation and management of the species.