SUMMARY: A search has been made for phages active on Clostridium perfringens, types A, B, C, D, E, and F. Twelve of 49 type A strains, 10 of 31 type B strains, 10 of 26 type C strains and none of either 38 type D, 5 type E or 3 type F strains were lysogenic. The ‘temperate’ phages obtained from these lysogenic cultures, together with similar phages obtained from crude material, such as sewage, were only active on strains of the same type as the lysogenic culture from which they were isolated; some of them were very host-specific. It was possible to induce partial lysis in lysogenic strains by ultraviolet radiation, nitrogen mustard and mercaptoacetic acid. Other phages isolated from crude material resembled the ‘virulent’ phages of aerobic bacteria. These phages lysed strains belonging to types A, B. C, D, and F. Despite a long search for phages that would lyse them, a high proportion of the strains of Clostridium perfringens examined remained insusceptible to all the phages isolated. A close relationship between phage susceptibility and colonial morphology was noted. Smooth and rough strains were usually phage susceptible; mucoid strains were usually phage-resistant.