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SUMMARYA salmonella phage which attacks only flagellated bacteria (Sertic & Boulgakov, 1936 b) has been studied. Tests with naturally occurring strains, and with artificial serotypes to which foreign H antigens had been transduced, have shown that sensitivity depends on the H antigen: bacteria with antigens of the g-complex are resistant, and with antigens 1.. ., e,h, or Arizona 13, are sensitive only to appropriate host-range mutants. Tests with non-motile and motile variants of the same strains showed that paralysed (non-motile H) as well as non-flagellated bacteria are resistant and thus that the flagella must be active as well as of correct antigenic type. Where resistance was due to absence of suitable flagella, i t was associated with impaired adsorption of phage. Removal of the flagella from a sensitive strain led to diminished adsorption; a similar result was obtained when the bacteria were artificially paralysed in various different ways. No adsorption to detached flagella was detected, probably because they were inactive. Adsorption of the phage led to immobilization and agglutination of the bacteria, probably by a direct effect on the flagella. Electron micrographs showed phage particles attached to flagella, and infection could evidently follow adsorption to distal parts of a flagellum. The genome of the infecting particle may perhaps reach the bacterial body by being injected into an active flagellum at the point of initial attachment, and then travelling inside the flagellum.