Alkoxysilane with phosphonium biocides as coupling agents were covalently attached to cotton-fiber surfaces. and the antibacterial activity of the surface-treated fibers against Staphylococcus aureus and Escherichia coli was evaluated by the viable cell counting method in sterile distilled water. These fibers with phosphonium salts were found to exhibit high antibacterial activity against S. aureus and E. coli, particularly against S. aureus, and the activity increased as hydrophobicity of the substituents bonded directly to phosphonium ions increased. Furthermore, morphological changes of the cells of S. aureus and E. coli in contact with the fibers were evaluated by scanning electron microscopy. It was found that the fiber with the phosphonium biocides exhibited bacteriostatic as well as bactericidal activity against both strains, which was evident from observation of normal and deformed cells of these species in contact with the fibers.