A possibility of detection of atoxigenic clones in the cultures of Cl. botulinum, types A and B, under conditions of treatment with acridine strains, UV-irradiation and heating of spores was studied. Of the 10 strains investigated, atoxigenic variants were obtained only in 3, belonging to type B; the frequency of formation of atoxigenic clones under the effect of ethidium bronide constituted 36--45.7%, and of the UV-irradiation-4.2%; heating of spores proved to be ineffective. A frequent loss of the toxigenicity sign coincided with the view on the elimination of the episomal determinant of the toxin-formation, not with the mutational nature-of this change. An association between the differences in stability of retention of the toxigenicity signs in various strains and types of Cl. botulinum and possible differences of the corresponding genetic determinants of the toxin formation is discussed.