Reports of hybridization between Macaca tonkeana and Macaca hecki were investigated in Central Sulawesi, Indonesia. We defined sets of morphological traits that were diagnostic for M. tonkeana and M. hecki and then located an areas where animals had intermediate or mosaic features. Hybridization as indicated by morphology was detected between M. tonkeana and M. hecki. The hybrid zone appeared to be strongly centered at the road that crosses the isthmus of Central Sulawesi from Tawaeli to Toboli. Macaques in this region were not morphologically uniform; animals from the western area of the Tawaeli-Toboli road resembled M. hecki, while animals from the eastern area resembled M. tonkeana. The hybrid zone was found to be smaller than previously thought, with maximum dimensions of approximately 15 and 7.5 km. Clines for diagnostic morphological features were broadly coincident, suggesting that the hybrid zone originated by secondary contact. Analysis of three museum specimens collected in 1916 provided evidence that the hybrid zone has been in existence since at least then. The narrow width of the hybrid zone, along with its age, suggested that some prezygotic or postzygotic barrier must exist to full introgression between M. tonkeana and M. hecki.