Sexual discrimination of Hyphantria cunea Drury (Lepidoptera: Erebidae) is very easy through checking male and female antennae of adult moths. Often sexing of insects in pupal stages is also needed. Although weight and size of male and female pupae are quite different, these alone are not enough to distinguish the sexes in every instance. Examining pupae of H. cunea with a binocular microscope reveals differences between the male and female on the posterior ventral surface of the abdomen. The genital slit is located on the anterior edge of the 8th abdominal segment of the female while it is located in the mid-part of the 9th abdominal segment in the male, on the ventral surface. Also, the genital slit is about twofold longer in the female than in the male.