Although the Mayr’s definition of species is commonly accepted for eukaryotic organisms, bacteria do not meet those established criteria. Thus, a special approach is necessary to elaborate the definition resistant to such limitations as for example the lack of sexual reproduction or horizontal gene transfers. One of the most problematic taxa in microbiology is the Bacillus cereus group composed of several closely related bacteria. Thus, on the basis of this model, doubts concerning bacterial taxonomy are discussed in the light of actual, molecular data.