Background: Novel pathogens can emerge into humans via one-step transmission from a reservoir host, an animal species in which the pathogen is maintained, or a two-step process in which the pathogen is transmitted from the reservoir host into a different amplification host species and thence to humans. Here we use serosurveillance and mathematical modeling to discover whether monkeys serve as reservoir or amplification hosts for mosquito-borne chikungunya virus (CHIKV). CHIKV invaded the Americas in 2013, and our study provides key data for predicting whether and where CHIKV will establish enzootic transmission among animal hosts in the New World. Results: Over three years we captured 219 African green monkeys, 78 patas monkeys, and 440 Guinea baboons, the three monkey species near Kedougou, Senegal. Monkey age was determined by anthropometry and dentition, and exposure of each animal to CHIKV was determined via detection of neutralizing antibodies. Age and exposure were used to estimate age-specific CHIKV seroprevalence, force of infection (FoI), and basic reproductive number (R0) in each species. CHIKV FoI were extremely high, ranging from 0.13 (95% CI, 0.07-0.22) in patas in 2012 to 1.12 (95% CI, 0.81-2.28) in African greens in 2011. R0 ranged from 1.5 (95% CI, 1.3-1.9) in patas in 2012, to 6.6 (95% CI, 5.1-10.4) in baboons in 2011. Conclusions: These findings demonstrate that monkeys in this region are constantly exposed to CHIKV transmission, even when population seropositivity, and therefore immunity, was too high for monkeys themselves to support continuous CHIKV transmission. We therefore conclude that monkeys in this system serve as amplification rather than reservoir hosts of CHIKV.