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In 1931 Hadley, Delves and Klimek described a filterable variant of B. dysenteriae Shiga which was distinguished by individual morphological and cultural characters and which they termed the G-type. Since that date a number of papers have appeared confirming their discovery either in whole or in part. The observations recorded in the present thesis are in almost perfect agreement with the statements of Hadley and his co- workers, although it must not be assumed that agreement extends to the details of interpretation. The G organism - derived in the present case from B. oaratyphosus B (Tidy) - proved to be a Gram-negative coccus growing only very slowly in broth and giving rise to minute colonies on agar; differing markedly from the parent culture in biochemical properties; relatively though variably heat -resistant; avirulent and non -toxic; and resistant to a strain of bacteriophage active against B. aratylhosus B. Over a considerable period of time the G organism tended to give off Gram-negative bacillary variants, all of which showed close biochemical relationship to the G form and to one another. No reversion occurred to the parent form. These features - and others - are discussed in detail in relation to the published work of Hadley and other authors. The discussion also includes an appraisement of the significance of these findings for the homogamic theory of the origin of bacteriophage (Hadley), and a section on the possible relation of filterable forms of bacteria to the problems of communicable disease.