Objective: Ultrasonic determination of fetal sex in ewes under field conditions by a single examination. Material and methods: Accuracy of transrectal and transabdominal ultrasonographic determination of fetal sex was examined in 210 ewes with singletons or twins between the 40th to 90th day post mating. Examinations were conducted under field conditions by ultrasonography with a linear-array 5 MHz transducer. Day 1 was the first day after mating. Single transrectal and transabdominal ultrasound examinations were performed in standing position of the sheep. Fetal sex determination was performed on days 40 to 49 (group I), days 50 to 59 (group II), days 60 to 69 (group III), days 70 to 79 (group IV) and days 80 to 90 (group V). Definite diagnoses were made after lambing. Results: The accuracy was 0% in group I (0/42), 40.0% in group II (13/42),33.3% in group III (14/42),64.2% in group IV (27/42) and 47.6% in group V(20/42). The differences between groups IV and V to groups I to III were significant(p < 0.05). The accuracy of fetal sex diagnosis was also influenced by singletons or twins (p < 0.05). Conclusion and clinical relevance:The results show that a single ultrasonographic examination is nota proper method for determination of fetal sex in sheep underfield conditions.