We cloned and sequenced the gene encoding the major outer membrane protein (MOMP) of two Chlamydia psittaci strains, guinea pig inclusion conjunctivitis (GPIC) strain 1, and meningopneumonitis (Mn) strain Cal-10. Intraspecies alignment of the two C. psittaci MOMP genes revealed 80.6% similarity, and interspecies comparison of C. trachomatis and C. psittaci MOMP genes yielded about 68% similarity. As found previously for C. trachomatis MOMP sequences, stretches of predominantly conserved sequences of GPIC and Mn MOMPs were interrupted by four variable domains whose locations were identical to those of C. trachomatis MOMPs. Seven of eight cysteine residues were found at precisely the same positions in GPIC, Mn, and C. trachomatis MOMPs, emphasizing their importance in structure and function of the protein. Collectively, these results indicate that C. psittaci and C. trachomatis MOMP genes diverged from a common ancestor.