The morphology and division ofEntodinium dubardi dubardi Buisson1923, found by the present writer in the rumen of a white‐tailed deer(Odocoileus virginianus borealis)from the state of New York, are described.E. longi‐nucleatumwas found for the first time in this host. A transitional form ofE. d. dubardileading toE. d. cerviSladecek 1946, was seen. Rotund or orbicular‐like individuals occurring in the material are the postdivisional stages ofE. d. dubardi.Comparative study of preparations from the mule‐deerOdocoileus hemionis hemionis, O. hemionus Macrotis, and another white‐tailed deer gave the same results as in the study of the principal host. Very close resemblance of the post‐divisional stages ofE. d. dubardiwithE. bovisWertheim andE. orbicularisBush&Kofoid is noted. New data have been established to regardE. simplexDogiel as a synonym ofE.