After 24 transfers in embryonated eggs a strain of variola virus (Chinese) was established in the testis of the rabbit and maintained for 11 passages at intervals of 7 days. Residence in the rabbit testis was not accompanied by any significant alteration in the species identity of the virus. A second strain of variola virus (Minnesota) was transferred 180 times in embryonated eggs with no apparent change in its behavior. Subsequent attempts, however, to maintain this strain in the rabbit by serial testicular passage were unsuccessful. These observations are discussed in relation to the so called transformation of variola to vaccinia virus by animal passage.