Abstract
A phage which attacks many motile strains of the Proteus-providence group did not replicate on aflagellate or paralysed mutants. The phage consisted of an hexagonal head, a tail and one tail fibre and resembled x1. Electron micrographs demonstrated adsorption of the phage to the flagellar shaft of the host and the presence of phage at the flagellar base. Some of the latter phage possessed empty heads, and we postulate that the phage injects its nucleic acid at the base. This phage and x1 had a slight serological relationship.