Chlamydiae are of medical interest because of their characteristic developmental cycle with the associated features of intracellular parasitism, and the diverse diseases they cause in man and animals. Currently, two recognized species compose the genus Chlamydia. 1 Man is the primary host of Chlamydia trachomatis, with the exception of the mouse biotype. Animals susceptible to C. psittaci infections are widely distributed in the animal kingdom, ranging from ectothermic vertebrates, wild and domesticated birds, and mammals to man. 2, 3 The goals of this discourse are to describe the diverse diseases caused by chlamydiae in different animal species and to illustrate pathogenetic events and lesion development following chlamydial infections.