Thirty eight strains of microorganisms of the pullorum-gallinarum group, of European, American or Brazilian origin were investigated bacteriologically in order to ascertain as to the fixity of their properties and the possibility of transforming a strain in another. a) Five types could be distinguished in our strains: 1 - gasogenic pullorum; 2 - non-gasogenic pullorum; 3 - intermedius; 4 - gasogenic gallinarum?; 5 - non-gasogenic gallinarum. The first, two and fifth types are well known and recognized by specialists; the fourth group has been recognized by Beck & Eber, in 1920; the third group was recognized by ourselves, in 1935. b) The table found in this paper summarizes the most important characteristics of these five types. The changes produced on neutral red, H²S production, fermentation of glycerol, isodulcitol, xylose, dulcite, sorbitol and maltose, the activity upon Seignette salt, gas production and appearance of the colonies, are among the characteristics on which the distinction of these five types is founded. c) The gasogenic gallinarum type, on account of its action on sorbitol and xylose, and also on Jordan's medium, must be rather considered as a gasogenic intermedius type. d) Other biological characteristics, such as activity upon milk, milk-whey, dextrin, etc., accepted by several specialists, appeared as of no practical value to the distinction of the different types of our strains. e) The strains kept their properties invariable during all the time of our investigations. The hypothesis of a possible transformation of one type into another has not been confirmed in any of the studied strains and this justifies the idea of independence of each one of the accepted types. f) Some discrepances in fermentation of maltose as found by several authors in microorganisms of this group, have been not confirmed in this paper. g) In blood horse serum a factor capable of transforming maltose was found making it fermentative. This substance is thermostable and resistent to some antidiastatic substances. h) The serological analysis of the studied strains did not allow a antigenic distinction among the different types of the bacterial group.