The development of “natural immunity” to homologous and heterologous ocular challenge with Chlamydia trachomatis was examined in cynomolgus monkeys given primary or secondary inoculations of C. trachomatis serovars B, C, and E. Overall, serovar E gave more severe disease than the other serovars. The amount of disease induced by the 2 serovar B strains examined, TW-5 and HAR-36, varied. In general, animals showed resistance to homologous secondary challenge, which was better characterized by a reduction in the recovery of culturable organisms than by a reduction in clinical disease. However, following initial heterologous challenge, neither resistance nor aggravation of disease was seen. These observations suggest that a trachoma vaccine will have to contain the serovar-specific antigenic determinants for multiple chlamydial serovars.