Chlamydia psittaci is a diverse group of organisms that affects birds and mammals. The number of biovars is unknown, and less is known about the number of serovars. Our restriction endonuclease analysis indicates that there are at least 5 biovars including avian, abortion-enteritis, IPA, M56, and GPIC. Monoclonal antibody studies revealed 4 serovars in the avian biovar. Monoclonal antibody studies have not yet been performed to identify multiple serovars in the other biovars; however, microimmunoassay studies indicate that a number of serovars may exist in the abortion and arthritis biovars. Of the 4 avian serovars, 2 are of major importance in the US avian population. These 2 serovars, psittacine and turkey, are each associated with important host preferences and disease characteristics. The turkey isolates have all been associated with either a serious disease in birds or human beings or with major epizootics in turkeys, often resulting in human disease. The psittacine serovar has been associated with serious disease in human beings; however, human involvement is usually limited to sporadic cases following exposure to companion birds or pigeons. The other 2 serovars, German duck and WC, are single isolates and their distributions and disease characteristics are not known.