Stable L-forms were for the first time obtained under the effect of tetracycline on the Mec thnikov vibrio and NAG vibrio. The process of conversion of the L-forms under the effect of tetracycline was the same as under the action of penicilline. Three types of the L-forms were obtained: stable tetracycline-resistant L-forms, stable tetracyclin-superresistant and tetracycline-dependent L-forms. Under the reversion of the unstable L-forms strains with a typical complex of biological properties and atypical revertants were obtained.