The newly described microorganism ‘Simkania Z’, related to the Chlamydiae, has been shown to be associated with bronchiolitis in infants and community acquired pneumonia in adults. The prevalence of infection in the general population is unknown. A simple ELISA assay for the detection of serum IgG antibodies to ‘Simkania Z’ was used to determine the prevalence of such antibodies in several population samples in southern Israel (the Negev). The groups tested included 94 medical and nursing students, 100 unselected blood donors, 106 adult members of a Negev kibbutz (communal agricultural settlement), and 45 adult Bedouin, residents of the Negev. IgG antibodies to ‘Simkania Z’ were found in 55–80% of these presumably healthy individuals, independently of antibodies to Chlamydia trachomatis and Chlamydia pneumoniae. The Bedouin had a seropositivity rate of 80%, while all other groups had rates of between 55 and 64%. These results indicate that ‘Simkania Z’ infection is probably common in southern Israel.