A prophage was induced from cells of the pathogenic spirochaete Serpulina hyodysenteriae using mitomycin C. Five to seven hours after mitomycin C was added (8 micrograms/ml, final concentration) to S. hyodysenteriae B204 cultures in BHIS broth (OD620 = 0.9) cell lysis was detected as a decrease in culture optical density. Bacteriophage particles attached to whole cells and to cell debris were detected by electron microscopic analysis of negatively stained (2% PTA, pH 7.0) bacteria harvested by centrifugation from mitomycin C treated cultures. The phage particles consisted of a head (45 nm diameter) and a tail (64 nm x 9 nm). Bacteria from untreated cultures lacked phages detectable by electron microscopy. The appearance of bacteriophage particles in mitomycin C treated cultures correlated with the appearance of extrachromosomal DNA, 7-8 kb in size as estimated by agarose gel electrophoresis, in DNA preparations from treated S. hyodysenteriae cells. When cultures of other S. hyodysenteriae strains (B78, B169, A-1, B8044, B6933, Ack300/8, R-1) and S. innocens 4/71 in BHIS were treated with mitomycin C (8-15 micrograms/ml, final concentration), phages similar in morphology and size to the S. hyodysenteriae B204 were induced.