Little is known about the filamentous Pseudomonas aeruginosa bacteriophage Pf and no information is available concerning the nature of its receptors on the host organism. In view of the fact that the filamentous coliphages infect by means of sex pili, it might be expected that Pf uses as receptors the polar pili present on its host. This paper describes experiments designed to establish whether or not this is the case. It is shown that pilus-free mutants of the host organism are resistant to Pf and that the phage does not adsorb to them. Electron microscopy, using specific antibody labeling to distinguish pili from phage, shows that the Pf filaments are attached preferentially at the pole of the cell after adsorption, the full 1950 nm length remaining visible. It is suggested that the virions adsorb to the tips of the pili which then retract pulling the Pf filaments until they contact the cell wall. It is also found that Pf-infected cells produce their progeny by extrusion from the poles without lysis.