1. A natural system of phage types ofSalm. paratyphi B has been described. 2. Each group comprises a number of different phage types arranged according to the presence in the strains of the three natural phages I, IV and VI. 3. The phage types belonging to each of the groups so far postulated have two characteristics in common: (a) all give identical lytic reactions (group reactions) to those typing phages that are specific adaptations of phage 1 ofFelix andCallow, and to bacteriophages found in mixed cultures; (b) all elaborate identical phages when grown in mixed cultures with strains belonging to one of the other phage types of a different group. 4. The subdivision of groups into types is based on the lytic reactions to phage preparations representing the natural phages I, IVa, IVb and VI (type reactions). The first three of these phages correspond, respectively, to the typing phages 3b, Taunton and Beccles of theFelix andCallow scheme. 5. The possibilities of extending the natural system and of its practical application are indicated. 6. The bearing of these findings on the problem of the nature of bacteriophage is discussed.