Corynebacteria (C. diphtheriae var. gravis, mitis, minimus, intermedius, C. belfanti, C. ovis, C. ulceraus) producing diphtheria toxin are found as variants sharply differing from one another morphologically (from long branching rods to coccobacilli), culturally, biochemically and by other signs. The toxin synthesis in them was determined by genetically heterogeneous prophages tox+; tox+ virus exchange is possible between the corynebacteria. Since production of specific toxin, the cardinal sign of C. diphtheriae species, is connected with the genoms of genetically heterogeneous viruses tox+, the circle of whose hosts is very wide, it can be supposed that there is no genetically homogeneous taxonomic category "diphtheriae species". Several species of corynebacteria--lysogenized (toxigenic) or capable of being lysogenized (nontoxigenic) with tox+ viruses are included into the composition of the corynebacteria groups which medicine considers as "species" for over 80 years. A possiblity of becoming a diphtheria causative agent is determined by the capacity of corynebacteria to become infected with the circle of viruses tox+ the DNA of which can become stably integrated with the gene of this bacterium. The mentioned approach to taxonomy is possibly applicable to microorganisms in which the pathogenicity signs (the toxin) were determined, as in C. diphtheriae, in the virus genoms (Bac. botulinus).