~_ _ _ _Chlumydiupneumoniae IOL-207 genomic DNA was hybridized with a 1.5 kb labelled DNA probe containing the 3' region of the coding sequence for the major outer membrane protein (MOMP) of C. trachomatis Serovar L1. An 8.5 kb BgAI fragment containing the complete MOMP gene was cloned into ilEMBL3. Two hybridizing EcoRI fragments were sub-cloned into the LZAP I1 cloning vector and the resulting plasmids were used as templates for sequencing both strands of the C.pneumoniae MQMP gene. Computer taxonomic studies using the nucleotide and inferred amino acid sequence of the MOMP of C. pneumoniae IOL-207 and all known chlamydial MOMP sequences supported the designation of C. pneumoniae as a new species, but electron microscope studies suggested that the presence of pear-shaped elementary bodies (EBs) may not be a reliable taxonomic criterion.