I examined prevalence of mandibular oligodonty in wild sheep (Ovis) and related it to evolutionary theory and classification. Inspection of 5,300 mandibles that included most subspecies of sheep revealed that this anomaly was widespread in New World sheep (subgenus Pachyceros) but was rare in Old World sheep (subgenus Ovis). Among the pachyceriform sheep, Siberian snow sheep (O. nivicola) had the highest prevalence of mandibular oligodonty (60.3%), followed by Dall's sheep (O. dalli, 23.0%), and bighorn sheep (O. canadensis, 10.1%). In Old World sheep, occurrence averaged 1.7% for argalis (O. ammon), 1.0% for urials (O. vignei), and 1.7% for mouflons (O. gmelini). Prevalence in domestic sheep was 4.3%. Overall, more recently evolved types exhibited the anomaly to a greater degree than did more primitive ones.