Serial transrectal ultrasonographies were performed measuring different structures of mouflon (Ovis gmelini musimon) fetuses to establish their gestational age. Data were obtained from 14 mouflon ewes that became pregnant after estrous synchronization. From day 25 of gestation onward, the crown-rump length and the trunk diameter of the embryo were estimated. Later, when the fetal organs and structures could be distinguished, several measurements were done in the skull (occipito-snout length and biparietal and orbit diameters), the trunk (thoracic diameter and width of vertebrae), and the abdominal cavity (stomach and kidney longitudinal diameters); femur length was also determined. All fetal measurements were correlated with gestational age (P < 0.001), but the parameters showing the highest correlation with time (R² up to 0.94) were the crown-rump and occipito-snout lengths and the thoracic and biparietal diameters. No significant differences were observed in pregnancies with one or two fetuses in the period studied.