Tamaraw, an endemic species on the Philippine island of Mindoro, is a critically endangered animal listed by IUCN. Although the population size of tamaraw has been monitored by the Department of Environment and Natural Resources of the Philippines annually from 1999, there is no academic report on the wild tamaraw population. Therefore, we investigated the present tamaraw population size and herd behavior in their natural habitat. The study area covered about 4,000 ha of natural grassland located in Mts. Iglit-Baco National Park, with 16 strategically located observation sites. The tamaraw population was counted using the Intensive Concentration Count Method or Simultaneous Multi Vantage Point Counts for five consecutive days in April, 2006. Fresh fecal samples of tamaraws were also collected from seven observation sites to determine prevalence of endoparasites. A total of 263 individuals were observed, consisting of 162 adult (62%), 52 juveniles (estimated ages of 1-4 years: 20%) and 49 yearlings (19%). Out of the 263 individuals, 29 (11%) were observed solitary, of which the majority were adult males (15/29). On the other hand, the remaining 234 tamaraws formed 71 groups consisting of 2-7 head per group. Out of 65 groups successfully sexed for adult animals, 63 (93%) had one or two adult cows with or without calves and 36 (55%) had one adult bull. From 15 fresh fecal samples, Coccidia eggs were detected in 10 cases and Fasciola eggs in 3 cases. These results demonstrated that 1) the tamaraw population is still in the critical ranges and there is a considerable deviation in the sex ratio of adult animals and in the proportion of juveniles to yearlings, 2) the tamaraw usually form families consisting of one bull with one or two cows, with the consequence of some solitary bulls, and 3) further analysis is required on the prevalence of endoparasites in the tamaraw.