The aims of this study were to evaluate the reliability, under general farm conditions, of the use of a single transrectal sonogram in pregnant mares to determine fetal sex by locating the genital tubercle, and the feasibility of extending the period of gestation in which this examination can be carried out. This research was done during routine reproductive examinations on three different stud farms. Forty mares between the 54th and the 89th day of gestation were examined once; gestation was calculated by identifying the last day of mating as Day 0. In order to verify the precision of the determination, data regarding the sex of the foals at birth were collected. This information was not available for two of the 40 mares examined. The diagnosis was made without any certainty level or any other methods which might quantify or increase the reliability of the examination. Of 40 mares, a diagnosis was possible in 35 (87%); in five mares (12%), it was not possible due to insufficient data. Twenty-six of 40 sex determinations (65%) were correct, 9 of 40 (22%) were incorrect. The comparison of the percentages obtained using the chi2-test was P = 0.0011. In spite of positive statistical analysis, the results of this study are not sufficient to allow systematic use of this approach due to the high percentage of error.