Semi-solid culture media, and more especially media rendered semi-solid by temperatures of from 30° to 40° C., seem to have an important bearing in the differentiation of bacterial species, particularly those presenting various degrees of motility. In such media not only the effect of differences in consistence on the motility of an organism may be noted, but the effect produced by various chemicals and nutrient ingredients on the growth and motility may be readily observed. By systematically varying the constituents of such media it has been possible to produce a medium in which the behavior of Bacillus typhosus differentiates it from the various members of the colon group; and also to produce a medium in which the colonies of Bacillus typhosus assume a form which distinguishes them from the colonies of the colon bacilli in plate cultures. Bacillus typhosus alone of all the organisms investigated during these experiments has displayed both the power of giving rise to thread-forming colonies in the plating medium and that of the uniform clouding of the tube medium, hence these two characters may prove to be of great value in the identification of this organism. The practical application of the use of these media has led to the ready detection of Bacillus typhosus and its isolation from the stools of patients suffering from typhoid fever. No suspected water has been subjected to test, but from the investigation of artificially infected tap-water the media here described may be assumed to have an application in the detection of Bacillus typhosus in such waters.