A strain of Chlamydia trachomatis was isolated from a patient with nongonococcal urethritis (NGU). Alternate passages between chick embryo and McCoy cell culture were examined. From the Giemsa stained coverslips taken from the cell culture 96 hours after inoculation, we found, to our surprise, that elementary bodies (EBs) distributed over a large area, and several intact cells embedded in them. These pure EB particles are round, fairly uniform in size and often appeared in pair. According to their morphology, distribution, arrangement and relationship with host cells, they are not the remains after cell lysis or directly released from host cells. We considered that they consisted of EBs which continued to divide by binary fission after their release. The name "Extracellular Multiply Form" was designated and their formation mechanism was proposed. This discovery gives a great challenge to primary theory, i.e. Chlamydias are obligate intracellular prokaryotic parasites. If we can further reveal the law of their formation, it will be of great significance both theoretically and practically.