The Author proposes a new classification of bacterium. It has been drawn up with the acquisitions of these last years regarding morfological, physiological and ecological characteristics and genetics of bacterium. It is also based on a new biological concept of the species, which is valid both for the "procarioti" and for the "eucarioti". Such a new concept, in time, makes a really new natural classification of the bacterium possible. Therefore the classification proposed here does not want to be nor must it be considered complete. It has been formed to be dynamic, capable that is of being modified as knowledge progresses.