Several type C and D strains of Clostridium botulinum, which had been converted to the toxgenic state by phages, were serially transferred through cooked meat medium with and without specific anti-phage serum. Most of the converted strains lost their toxigenicity even during transfer without antiserum, and the non-toxigenic variants that appeared were resistant to lysis and conversion by the original phage. However, in some combinations of phage and host bacteria toxigenicity was stable after ten transfers, though it showed a transient decrease, and the non-toxigenic variants that arose remained sensitive to lysis and conversion. When converted strains were transferred in medium containing anti-phage serum, toxigenicity was lost more rapidly than in the absence of serum and the non-toxigenic variants that appeared remained sensitive to lysis and conversion by the parent phage. Filtrates of the supernatants of culture fluids of strains transferred without anti-phase serum converted non-toxigenic strains to toxigenicity at varying rates.