Experimental trachoma in nonhuman primates has been handicapped because trachoma (as opposed to inclusion conjunctivitis) produces very mild clinical disease and minimal microbiologic changes. In a direct comparison between the Old World Taiwan monkey, Macaca cyclopis, and the New World owl monkey, Aotus trivirgatus, the latter was found to be much more susceptible. Owl monkeys manifested severe disease for a period of 2 or more weeks, while none was observed in Taiwan monkeys during the same period of time. Given identical inoculations of 2,000 EID50 in the left eye and 20 EID50 in the right eye, six Taiwan monkeys on the 7th day averaged 21 inclusions in the left eye and one in the right. Comparable figures for six owl monkeys were 641 and 1,396, respectively.