Ostertagia cf. kasakhstanica, the putative minor morphotype of the polymorphic ostertagiine O. bisonis, is reported in Bison bison from South Dakota. Descriptions of the synlophe and details of other diagnostic characters useful in identification and differentiation of this minor morphotype from other ostertagiines in wild and domestic ruminants from western North America are presented. Laterally, the cervical synlophe is consistent with Type Ib pattern among ostertagiines and is within the range of variation previously described for O. bisonis. Among recognized minor morphotypes of ostertagiines in wild and domestic ruminants from North America, O. kasakhstanica is distinct from O. kolchida, O. dikmansi, and O. arctica based on a tapering lateral synlophe. It is further distinguished from O. lyrata by the pattern of the cervical synlophe in the lateral field, structure of the genital cone and spicules, and dimensions of the esophageal valve. Although morphologically similar to T. davtiani/T. trifucata and T. boreoarcticus f. minor, it can be readily distinguished by the 2-1-2 bursal pattern, the structure of the spicules and genital cone, and the length of the esophageal valve. Ostertagia bisonis/O. cf. kasakhstanica circulates among pronghorn, mule deer, bison, and cattle in west-central North America.