The subfamily Zapodinae is known from Pliocene and Pleistocene deposits of North America and now occurs over much of northern North America and in Szechuan and Kansu, China. The living species occur among grasses and low herbs in damp or marshy places both in forested areas and in plains areas. The early Pliocene _Macrognathomys nanus_ Hall (1930:305), originally described as a Cricetid, is actually a Zapodid as shown by the structure of the mandibular ramus, shape of the incisors, and occlusal pattern of the cheek-teeth. If _Macrognathomys_ can be considered a member of the subfamily Zapodinae (possibly it is a sicistine) then it represents the oldest known member of this subfamily. Judging from the published illustrations, _Macrognathomys_ seems to be structurally ancestral to the Mid Pliocene _Pliozapus solus_ Wilson; the labial re-entrant folds are wider and shorter and on m2 and m3 fewer. The difference in stage of wear of the teeth in _Macrognathomys_ and _Pliozapus_ is a handicap in comparing the two genera but they are distinct. Wilson (1936:32) points out that _Pliozapus_ clearly falls in the Zapodinae and stands in an ancestral position with respect to the structurally progressive series _Eozapus_, _Zapus_, and _Napaeozapus_. Nevertheless, _Pliozapus_ cannot be considered as directly ancestral to _Eozapus_ because of the progressive features in the dentition of _Pliozapus_. Wilson (1937:52) remarked that if _Pliozapus_ is ancestral to _Zapus_ and _Napaeozapus_, considerable evolution must have taken place in the height of crown and in the development of the complexity of the tooth pattern. In contrast to Wilson's opinion, Stehlin and Schaub (1951:313) placed _Pliozapus_ and _Eozapus_ in the subfamily Sicistinae because certain elements in the occlusal pattern of the cheek-teeth are similar. I disagree with those authors and hold with Wilson; I consider _Pliozapus_ and _Eozapus_ in the subfamily Zapodinae. In dental pattern _Pliozapus_, as Wilson (1936:32) pointed out, resembles the Recent Eurasiatic sicistid, _Sicista_ more than do _Zapus_ or _Napaeozapus_. Nevertheless, from _Sicista_ Wilson distinguishes _Pliozapus_ and relates it to the subfamily Zapodinae by: "more oblique direction of protoconid-hypoconid ridge, anterior termination of this ridge at buccal portion of protoconid rather than between protoconid and metaconid as in _Sicista_; cusps more compressed into lophs; cheek-teeth somewhat broader; greater development of metastylid; greater development of hypoconulid ridge, ... absence of anteroconid...."