The F-pilus retraction theory suggests that F-pili retract on contact with a recipient bacterium or pilus-specific bacteriophage, but there has been no direct demonstration of any forin of pilus retraction. Electron microscopy is used here to study average length changes in Pseudomonas aeruginosa RNA-phage pili after the adsorption of RNA-phage virions to the sides. The theory predicts that, if one phage per pilus is adsorbed, there should be a reduction of about 50 % in the mean length due to the adsorption of virions at random points; retraction would be stopped at the point of adsorption when the virus reached the bacterial surface.A phage-resistant, pilus-bearing P. aeruginosa strain (PAO~~), used as a control, showed no such change. The phage-sensitive strain PAOI showed an average reduction in pilus length of about 42'5% relative to its length before adsorption, or 50% relative to the ~~0 6 8 average. This strongly suggests pilus retraction in PAOI but not in ~~0 6 8 .In electron micrographs, phage virions are seen almost always at the bases of PAOI pili, whereas virions adsorbed to ~~0 6 8 are randomly distributed along the pili.