AbstractA strain of nonsymbioticA. proteuswas infected with endosymbiotic bacteria isolated from another strain of amoeba which had become dependent on the symbionts after a few years of spontaneously established symbiosis. In the newly infected amoebae, the bacteria avoided digestion and multiplied at a faster rate than the hosts, reaching the maximum carrying number (about 42,000 per amoeba) in fewer than ten cell generations of the hosts. The experimentally infected amoebae were also examined under the electron microscope, and the development of bacteria‐containing vesicles was followed. The results show that the infective bacteria that were initially harmful to host amoebae have become harmless and that they have changed in their mode of multiplication during the course of establishing a stable symbiosis with their host