Bacteriophage phi 6 infects its host, the Gram-negative bacterium Pseudomonas syringae, by a protein-targeted fusion of the virus envelope with the host outer membrane. In this investigation we present results suggesting that the phage nucleocapsid penetrates the host cytoplasmic membrane via a membrane invagination and an intracellular vesicle. This indicates that the prokaryotic plasma membrane might be more dynamic and have more common features with eukaryotic membrane systems than previously expected. Most of the nucleocapsid surface lattice protein is degraded in the cell, and the nucleocapsid core particle containing the viral dsRNA segments and the proteins necessary for the viral RNA polymerase activity can be isolated from the infected cells. The penetration is dependent on the energized state of the host cytoplasmic membrane. About 25% of the entering core particles are re-used in the progeny viruses.