[Aim]  To investigate methods of identifying the sex of the larvae and pupae of four key lepidopteran pests, Spodoptera frugiperda, Spodoptera litura, Ostrinia furnacalis and Chilo suppressalis. [Methods]  Morphological characteristics of the testes and ovaries of the larvae of the four pest species, as well as male and female pupae, were compared. [Results]  The testes and ovaries of the larval stages of all four pests were located near the dorsal surface in the second to third abdominal segment. The testes are juxtaposed in a kidney-shaped structure and the ovaries are juxtaposed in an elliptical structure during the larval stage. [Conclusion]  The presence or absence of testes was sufficient to determine the sex of larvae. Female pupae had an ovipositor orifice and gonopore connected into a short slit, whereas male pupae had two semicircular tumor-like projections on the gonopore. Therefore, male and female larvae of all four pest species could be distinguished by dissection to determine the presence or absence of testes, and male and female pupae could be distinguished by observing the presence, or absence, of a pair of semicircular tubercle-like projections near the caudal gonopore. Identifying the sex of larvae and pupae is useful for prediction and forecasting, and for laboratory research.