Laboratory of Physical Anthropology, Facul Muscular anatomy of the tail has rarely been studied in cercopithecoids. An important exception is Howell & Straus’ (1965) work on the rhesus macaque. However, tail muscular anatomy is only briefly described in their comprehensive work, which covers the whole body musculature. I investigated the origins and insertions of caudal musculature in five cadavers of the Formosan rock macaque (Macaca cyclopis) and two cadavers of the rhesus macaque (Macaca mulatta). The results of the rhesus macaque examination showed two major differences from the description of Howell & Straus (1965). In the present study, M. flexor caudae brevis was absent, and MM. iliocaudalis and pubocaudalis were found to insert more proximally than previously reported. The reason for these differences is not clear. However, intra-specific variation in tail length among regional populations of rhesus macaques may deserve consideration. Although the Formosan rock macaque and rhesus macaque do not show any significant differences in the origins of their tail muscles, they show great differences in the insertions of several muscles. Among the flexors, the insertions of three muscles (M. iliocaudalis, M. pubocaudalis, M. ischiocaudalis) were more distal in the Formosan rock macaques. Among the extensors, M.abductor caudae lateralis inserted into an additional distal caudal vertebra, and two muscles (M. extensor caudae lateralis M. abductor caudae medialis) became tendinous at more distal levels [by 1∼3 segment(s)] in the Formosan rock macaque. More distal insertions in the Formosan rock macaque are not surprising given its greater tail length. However, it is worthy to note that the difference is more marked in the extensors compared to the flexors. This may suggest that the extensors have a greater functional role in tail use among cercopithecoids.