A large quantity of avian tubercle bacillus, avian A strain which could not multiply on the plain broth agar media, was trans-inoculated on the plain broth agar media and was successively cultivated on them. By this procedure a variant strain was obtained which could multiply on the plain broth agar media. The author tried the comparative studies on the various natures of the original and varient strains, and obtained the following results: 1) The multiplying rate of the variant strain on the plain broth agar media is worse than that of the original strain on the glycerine media. 2) The shape of the variant is full of diversity, and coccoid ones were particularly observed in large numbers. 3) The acid-fastness of the variant strain is remarkably diminished. 4) The variant strain showed a marked fall of resistance to phenol and caustic soda. 5) In the absence of any subatrate, the reduction rate of methylene blue is consistently speedier in the variant strain, while, as for the oxygen consumption, the endogenous respiration of the variant strain is remarkably lower than that of the original strain.