Between August and December 2010, we studied the population density, structure, composition and home range of Alouatta seniculus (Red Howling Monkey) in the Alto Chibogado and Crucito (Gaita Island) located at the buffer zone of URRA reservoir, Tierralta, Cordoba, Colombia. Our results indicated the presence of two groups of A. seniculus at Chibogado locality, with five and six individuals respectively, and composed of three males, six females and two individuals of undetermined sex with densities of 71.89 ind./km 2 and 13.07 groups/km 2 . For Crucito (Gaita Island) we report the presence of a group of ten individuals composed by four females, two males and four individuals of undetermined sex with a density of 76.92 ind./ km 2 and 7.69 groups/km 2 . At Chigobodo, the male/female proportion was 1:2. For the Island group it was observed a greatest social interaction between adult females and immature (1:1.33). The Island group also presented the greatest home range (2.66 ha), followed by the groups of Chigobodo 1.93 ha and 0.77 ha, showing a direct relationship between group size and home range, and suggesting a greater availability of resources in the island system.