Roles of the bacterial genetic components in the endocellular symbiosis of xD strain of A. proteus were determined by application of plasmid curing agents and antibiotics. The effects of plasmid curing or blocking prokaryotic gene expression were analysed by two dimensional gel electrophoresis, autoradiography and experimental infection. Curing plasmids of the bacteria resulted in loss of the ability to infect the new host. By selective suppression of the transcription or translation of the symbiotic amoebae it was possible to localize the gene of xD strain specific protein in the bacterial chromosome. When the gene expression of bacterial chromosome was blocked, the symbiotic vesicles under went degradation in xD strain and the bacteria were unable to develop the symbiotic vesicles in the newly infected host. Thus the development and maintenance of the symbiotic vesicle are under the control of the bacterial genome. Plasmids of the symbionts were found to be essential for the initial step of symbiosis by enabling the bacteria to avoid digestion in phagolysosomes.