Based on 440 skulls recently collected from two areas of the wild ass population in Mongolia, the time course of tooth eruption and replacement was investigated. The dentition pattern allows identification of age up to five years. We also conclude that annual lines in the tooth cementum can be used to determine the age in years for wild asses older than five years after longitudinal tooth sections were made with a low-speed precision saw. The first upper incisor proved to be most suitable for age determination, although the starting time of cement deposition is different between the labial and lingual sides of the tooth. The accurate age of the wild ass can be determined from the number of annual lines and the time before the first formation of the cementum at the respective side of the tooth.