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The montane forests in Colombia have suffered strong degradation due to human activities; currently, most of them are remnants isolated and immersed in anthropogenic habitats. The red howler monkeys have persisted in these conditions but little is known on their biology and response to the alteration of their environment. During five months in 2005, we evaluated the population density and status of this species in the basin of the<br />
Barbas River (Quind&iacute;o and Risaralda departments), a low montane isolated forest fragment of 790 ha. We estimated density by censuses for linear repeated transects. Additionally we followed troops of howlers and evaluated vegetation variables according to howler ecological requirements. We estimated a density of 11.7&plusmn;6 groups and 132.1&plusmn;68 individuals per km², higher than what is usually reported for this species in continuous forests. This high density might be caused due to the isolation of the fragment that reduces the possibilities of dispersion of the howlers. The average troop size was 11&plusmn;2.5 individuals, with more than five males and seven females, and one to four immature individuals. In general, the forest offers to the howlers, good availability of food and structural resources. Currently, this population is not threatened by anthropogenic activities. Recently, this fragment has been protected as a Regional Park, and it exists a project to develop corridors of connection with others remnants. Nevertheless, this howler population is isolated from other populations, and therefore urgent measures of landscape management are needed to attenuate the effects of the isolation.</p>