Early stages of infection of Escherichia coli with the filamentous bacteriophage f1 were examined in the electron microscope. Purified phage-bacteria complexes were prepared at various time intervals after the initiation of synchronous infection. Cells were scored for the total number of F pili, the number of F pili with f1 attached, the number of intact phage particles which occurred at the surface of the cell, and F pilus length. Electron microscope autoradiographs were also prepared at each time interval. The results showed that the average number of F pili with f1 attached decreased with time as phage deoxyribonucleic acid (DNA) entered the cell. Concomitant with this loss, the remaining F pili became shorter. The rate of entry of phage DNA into the cell followed, with a short lag, the rate of loss of F pili with f1 attached. During the lag period, intact phage particles accumulated at the surface of the cell. The results from radioautographs showed that no phage DNA could be located within the F pilus. These results suggest that F pili are resorbed by the cell during infection with the bacteriophage f1. Parallel experiments with noninfected cultures further suggest that pilus resorption may be a normal cellular phenomenon.