The presence of spider monkey (Ateles geoffroyi) has been verified in the northeast of the state of Oaxaca. However, the information to assess their conservation status is scarce. This study focused during one year on the population census and habitat use of A. geoffroyi in a well-preserved fragment of evergreen forest. We used the distance sampling in line transects method to estimate the density of this primate, and instantaneous scan sampling to collect data of grouping, diet and activity patterns. We estimate a density of 84 spider monkeys/km 2 , which would imply a population size of 586 spider monkeys in a protected area of 7 km 2 , which is part of a continuous forest area of 41 km 2 . This population density is one of the highest reported for the genus Ateles, particularly for the species A. geoffroyi. The average subgroup size was 3.5 independent individuals and their most frequent composition was mixed, with one or more adult females, some with their infant, and one or more independent males. Spider monkeys dedicated a similar proportion of their daylight time to moving (30%) and feeding (22%) as they did to resting (48%). Diet was mainly frugivorous (84% of feeding time) including 30 plants morphospecies. These results suggest that the spider monkey population we have studied is potentially viable.