The geographic distribution of two congeneric monkeys (Cebus albifrons and C. apella) was studied in El Tuparro National Park in the llanos of eastern Colombia. Extensive trips were undertaken by land and river to collect 268 precisely located groups of these two species. The distributions of these monkeys were generally mutually exclusive and parapatric, contrasting with their known sympatry in closed-canopy rainforest farther to the south. The replacement of one species by the other was abrupt along the northern water courses in the park; however, the two species' ranges partially overlapped and contained islands of sympatry towards the southern parts of the park, where forests are more extensive. Along the rivers, C. albifrons was found in seasonally inundated forests and woodlands that were seldom entered by C. apella. Such inundatable vegetation was extensive in downriver sites in the typically flat llanos geography of this part of Colombia. Although some differential use of habitat was demonstrated, many forests containing the two species seemed essentially identical so that historical precedence, competition, or both may also be posited to partially explain the distributions, although this remains to be demonstrated.