The enveloped dsRNA bacteriophage phi 6 uses the pilus of Pseudomonas syringae as its receptor. It enters the host cell by fusion of the virus envelope with the host outer membrane, followed by penetration of the cytoplasmic membrane by the phage nucleocapsid. In this investigation we quantitated the adsorption and penetration of phi 6wt and a host range mutant, phi 6h 1s, to five bacterial strains. Adsorption rate constants were measured for the different phage-host combinations, the constant for phi 6wt with the standard host was 3.3 X 10(10) ml/min. Infections with 14C-labeled phage at different phage/cell ratios were used to measure the numbers of adsorbing and entering virions/sensitive cell. At high phage/cell ratios (200-250) the standard host adsorbed on the average 35-40 wild-type virions/cell, the saturation level being somewhat higher. It was shown that at phage/host cell ratios of 0.1-1 practically every virion produces an infectious center. The average number of entering phage particles per infectious center reached saturation around the phage/cell ratio of 50 and did not exceed 3 for the standard host. The phi 6 preparations used in this study had a specific infectivity of 0.7-0.9.