The authors investigated 286 strains of the tularemia microbe isolated in a number of countries of the Old and the New World from humans, hares, rodents, ticks, water and other objects, including 217 strains from the USSR and 69 strains from other countries. As a result, the earlier observed subspecific differences in the biological properties of the tularemia strains isolated in different regions were reconfirmed on an extensive material. It is proposed to designate the three subspecies distinguished within the species as follows: Francisella tularensis holarctica Ols., occurring in the Old World as well as in the New World; F. t. nearctica Ols., known only in North America, and F. t. media-asiatica Aikimb., occurring in Central Asia. Three biological varieties are distinguished in the holarctic subspecies: biovar japonica Rod., occurring on the Japanese islands, biovar I eryS (erythromycin-sensitive) distributed in Old and New World and biovar II eryR (erythromycin-resistant) known only in a few localities in Europe and Asia. Subspecific taxons differ from each other in several stable signs including biochemical-fermentation of glycerol and citrulline, and others, in the degree of pathogenicity for man and domestic rabbits, in the ecology and also in the area of distribution while the biovars differ in one sign and partly in the area of distribution. No differences were found between the North-American strains of the holarctic subspecies and the Eurasian holarctic strains of biovar I. The absence of serological differences between the individual taxons is no obstacle to infraspecific taxonomy of the tularemia microbe.