The peroneus tertius muscle was observed in five legs (2.9%) of three male specimens (3.4%) out of 174 legs of 87 (45 male and 42 female) crab-eating monkeys (Macaca fascicularis). They were all supplied by the deep peroneal nerve. Two types were observed in three legs of two specimens and grouped into Types III or IV and VIII or IX as classified by Kaneff (1980). These comprise a tendinous or a muscular expansion of the lateral compartment of the extensor digitorum longus muscle. While the muscle observed in both feet of another specimen seems to correspond to the “so-called peroneus tertius” reported by Sommer (1907)and Preuschoft (1961) in gorillas.