An electron microscopic investigation was performed on 28 Clostridium difficile strains isolated from 15 antibiotic-associated diarrhea cases and from 13 healthy infants. Through the use of supernatants of the cultures induced by mitomycin C (1 or 3 micrograms/ml), 18 of the 28 C. difficile strains proved to harbor phage particles with a different morphology and size. Most of the phages revealed belonged in phage group B1 with a non-contractile tail. Seven of the 11 toxin-producing, lysogenic C. difficile strains carried a defective phage structure (120 nm-long tail with an incomplete head capsule) alone or together with other normal phages. With different C. difficile strains as indicator, plaque formation could not be detected in any of the lysates.