Anti-Müllerian hormone (AMH), responsible for the regression of Müllerian ducts, is strongly expressed by eutherian fetal and postnatal Sertoli cells. Both AMH and testosterone levels are high during the period of fetal reproductive tract virilization which occurs largely in utero in eutherian mammals. Taking advantage of the fact that differentiation of the urogenital tract occurs after birth in marsupials, we studied the ontogeny and regulation of <i>AMH</i> in the tammar wallaby testis and related it to the expression of the androgen receptor in Sertoli cells. Testicular AMH expression was high between days 10–30 post partum, then fell to basal levels by day 60 and remained low until day 90, the oldest age examined. AMH expression was repressed by treatment of male pouch young with the potent androgen androstanediol. Thus, in the tammar, AMH expression decreases in response to androgen at the time of initial urogenital masculinization, in contrast to the situation in humans in which AMH is repressed by testosterone only at the time of puberty. The difference might be explained by the timing of androgen receptor expression which appears in tammar Sertoli cells at around day 40 of pouch life but only at a later developmental stage in eutherians.