A natural infection of T. axei in a horse was experimentally transmitted to lambs and calves both directly and indirectly. In sequence, following the infection of a calf and 2 lambs, this strain of T. axei again matured in the horse. Domestic rabbits were success-fully infected with 5 strains (3 equine, 1 ovine, and 1 bovine) of T. axei initially encountered as natural infections in these hosts. Exposures with the bovine and one of the equine strains also developed to maturity in both guinea pigs and hamsters, although the limited experience suggested that these animals were not so suitable hosts as the rabbit. No evidence of maturation was found in rats exposed to larvae of the bovine strain. T. colubriformis, which accompanied the ovine strain of T. axei, also matured in rabbits.