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ABSTRACTPathogenically diverse Chlamydiaspp. can have surprisingly similar genomes. Chlamydia trachomatisisolates that cause trachoma, sexually transmitted genital tract infections (chlamydia), and invasive lymphogranuloma venereum (LGV) and the murine strain Chlamydia muridarumshare 99% of their gene content. A region of high genomic diversity between Chlamydiaspp. termed the plasticity zone (PZ) may encode niche-specific virulence determinants that dictate pathogenic diversity. We hypothesized that PZ genes might mediate the greater virulence and gamma interferon (IFN-γ) resistance of C. muridarumcompared to C. trachomatisin the murine genital tract. To test this hypothesis, we isolated and characterized a series of C. muridarumPZ nonsense mutants. Strains with nonsense mutations in chlamydial cytotoxins, guaBA-add, and a phospholipase D homolog developed normally in cell culture. Two of the cytotoxin mutants were less cytotoxic than the wild type, suggesting that the cytotoxins may be functional. However, none of the PZ nonsense mutants exhibited increased IFN-γ sensitivity in cell culture or were profoundly attenuated in a murine genital tract infection model. Our results suggest that C. muridarumPZ genes are transcribed—and some may produce functional proteins—but are dispensable for infection of the murine genital tract.