Transmission electron micrograph of bacteriophages (phage f1 & f2) on a pilus of the bacterium ^IEsche- ^Irichia^i ^Icoli.^i Bacteriophages are viruses which infect only bacteria, some specific to one species of bacterium. Phages f1 & f2 occur only on male F-pili of ^IEscherichia coli.^i An F-pilus (containing the F plasmid) is a filament protruding from the wall of a male bacterium that is used in reproduc- tion. Here, an F-pilus runs diagonally at lower left. Many RNA phage f2 (spherical) are attached to it. DNA phage f1 appears as a long strand, and here two such phages are seen (centre, upper left) attached specifically at the tip of the pilus. Mag: x19,200 at 8x10, x2,740 at 35mm size.