Fusiformis nodosus was shown to cause severe footrot in sheep without the intervention of either, or both, of the other bacteria usually present in natural cases, namely, Treponema penortha and a motile fusiform bacillus. The progress of experimental infections in the sheep's foot with pure cultures of F. nodosus has been related to the histopathological changes observed. The organism was shown to invade the stratum lucidum of the hoof but did not penetrate beyond the granular layer. The epithelial cells of the stratum lucidum became detached and disintegrated. The horn became separated from the underlying tissues in this region. Penetration into the stratum corneum was accompanied by sinus formation and degeneration of the horn.