The difficulties encountered in distinguishing pneumococci from streptococci, particularly S. salivarius, have been enumerated and the morphological and cultural similarities between the two groups of organisms have been pointed out. If bile-solubility is taken as the distinguishing mark of a pneumo-coccus it will be found that there are a large number of bile-insoluble streptococci commonly met with in the respiratory tract and sometimes playing an important part in disease, which bear very strong resemblances to pneumococci both in shape, colony, and broth culture. The similarities are specially confusing in mixed cultures on agar plates and a number of the routine tests applied quite failed to distinguish them. There naturally follows the corollary that some of these tests fall wide of the mark, and they should therefore be left out of the bacteriologist's armamentarium when the identification of pneumococci is under consideration. Morphological and cultural properties are insufficient guides to species, while the same is true of the widely recommended tests of inulin fermentation and capsule formation. Not only do some pneumococci fail to ferment inulin, but so many strains of S. salivarius show active inulin fermentation that, in the present series of organisms taken at random, the incidence of inulin fermenters amongst S. salivarius is actually higher than amongst pneumococci. Capsules were found in all the strains of streptococci in which they were sought and in all the pneumococci investigated, so that encapsulation may be regarded as a property common to members of both species.