An electron microscopic survey of the occurrence of bacteriophages which appear spontaneously in cultures of haemolytic intestinal spirochaetes of human and animal origin was made. Excluding one isometric tailed phage particle which was observed in the form of free particle in proximity to a spirochaete of the w beta HIS strain HRM18, bacteriophages were never observed while examining cells of 21 weakly beta-haemolytic human intestinal spirochaetes (w beta HIS), swine Serpulina pilosicoli strain P43/6/78, and the avian strain 1380, although 50-100 cells of each spirochaetal strain were analysed. Isometric tailed bacteriophages were found associated with only three out of the 100 cells of strongly beta-haemolytic swine Serpulina hyodysenteriae strain P18A comparatively analysed. According to our results and previous published reports, the occurrence of bacteriophages which appear spontaneously in cultures of intestinal spirochaetes is a rare event.