Phthalate esters are commonly used as plasticizers for polyvinyl chloride and are known to be hormone-disrupting chemicals. We previously reported that mono-n-butyl phthalate (MBP) administered to rat fetuses induced cryptorchidism postnatally. The aim of this study was to investigate the biological effect of MBP on the transabdominal migration of the testis in prenatal rats by comparing this with the prenatal effect of the antiandrogen flutamide on testicular descent. Time-pregnant Wistar King A rats were divided into three groups: group I rats (N = 3) were administered MBP 0.3 g/day by gavage from gestational days 15 to 18; group II rats (N = 3) were injected with flutamide (30 mg/day) from gestational days 15 to 18; group III rats (N = 3) were administered solvent as controls. On the 19th gestational day, all rats underwent a cesarean section and the male fetuses were dissected to examine the position of the testis, which was significantly higher in the abdominal cavity in the MBP-treated rats than in either the flutamide-treated or control rats. No significant difference was observed in the position of the testis between the flutamide-treated and control rat fetuses. Our findings suggest that maternal MBP prevented transabdominal migration of the testis in prenatal rats, which may not have been due to either an antiandrogenic or estrogenic effect of MBP, but to a direct toxic effect of MBP on the testis.