Abstract An eight-year-long census and habitat evaluation of the Macaca sylvanus population was conducted in a 484-km2 area of the central region of the Middle Atlas Mountains in Morocco between June 1994 and October 2002. The authors walked a 93.5-km circuit – divided into 16 transect segments – 30 times with teams of trained research assistant volunteers, collecting data on a total of 2,805 linear km. Previous studies had reported an average density of 44–70 individuals per km2, while data from the present study indicate a progressive population decline, from 25 to 30 individuals per km2, down to a current average density of 7–10 I/km2. The population decline is attributed to the loss of prime habitat, mainly cedar forest, which has significantly decreased from 1994 to 2002, due to the growing impact of overgrazing by mixed flocks of goats and sheep and consequent forest degradation. At present, human-caused habitat deterioration in the Middle Atlas risks further compromising the future of the world’s only remaining large M. sylvanus population.