One liter of rumen fluid containing 4.7 × 10⁴ ciliates/ml, representing four genera including nine species of ciliates from a Japanese sika deer was inoculated into two unfaunated Japanese shorthorn calves. Two weeks after inoculation, all species originally present in the inoculum were subsequently detected in the rumen fluid of one or both calves. Ciliate densities ranged from 10⁵–10⁶ cells/ml over the remainder of the 33-wk experiment. The inoculum contained Diplodinium rangiferi. which lacks caudal appendages, as is characteristic for the species. However, three weeks later, the rumen fluid of both calves contained D. rangiferi, which possesses caudal appendages varying from a single spine to multiple spines with a complicated furcate appearance. The caudal spines of D. rangiferi did not disappear during the experiment, even when the diet of the calves was switched to the ration of sika deer from which the inoculum was obtained.