The morphology of a recently isolated strain of Chlamydia pneumoniae, YK-41, was compared by electronmicroscopy with C. pneumoniae TWAR, Chlamydia trachomatis L2/434/Bu and Chlamydia psittaci Cal 10. The results showed that "pear-shaped" morphology was not typical of C. pneumoniae. Basic morphological features, such as surface projections and hexagonally arrayed, regular structures in the inside layer of the outer membrane of elementary bodies, were very similar in these strains. The structure of strain YK-41 was identical with that of C. trachomatis and C. psittaci, but the profiles of elementary bodies were different from those of C. pneumoniae TWAR strains.