Bacillus piliformis (Tyzzer) was isolated from the liver of rabbits with Tyzzer's disease and serially passaged in embryonated hens' eggs. Weanling rabbits given the 32nd egg passage developed lesions typical of Tyzzer's disease and died. B. piliformis was reisolated from the liver of these rabbits in embryonated eggs. Outside the host cell, the motile vegetative phase appeared to be unstable, and no means was found to preserve its viability; the results of titrations were believed to be dependent upon the resistant stage or spore. The spore withstood repeated freeze and thaw and was resistant to heat treatment of 56 C for 1 hr but not 80 C for 0.5 hr. None of several antibacterial substances tested in embryonated eggs was completely inhibitory; B. piliformis was resistant to sulfamethazine and chloramphenicol. The taxonomic position of this pleomorphic, gram-negative, sporeforming, pathogenic bacterium which appears to grow only in certain cells of several species remains unresolved.