Susceptible pregnant sheep were infected with Chlamydia psittaci at different stages of early and mid-gestation. Placentas and foetuses were examined by microbiological, serological, histopathological and immunochemical methods at various intervals thereafter. While ewes were susceptible to infection from early gestation onwards and infections of placentas and foetuses occurred from about 60 days gestation (dg), pathological changes were not detected until after 90 dg. These changes consisted of initial rapid replication of C. psittaci in the trophoblast at the limbus of the placentomal hilus, leading to local necrosis and contiguous spread of infection to involve the cotyledonary and intercotyledonary placenta and apposing endometrium. Foetal pathology consisted of focal necrosis in liver and other organs suggestive of embolic dissemination of infection from the placenta. An early foetal immune response was detected including the demonstration of IgM and IgG antibody to C. psittaci and quantitative evidence of a B cell response to infection. Pathogenetic mechanisms are discussed.