Formoguanamine hydrochloride (FG) is known as a potent chemical to induce blindness in chick eyes by disrupting the pigment epithelium and visual cells in the retina. In this study, we examined the effect of FG on the structure and function of the compound eyes of the butterfly, Papilio xuthus (Lepidoptera, Insecta). We administrated 2 mg FG per 1 g body weight of the pupa at about the first one third of the whole pupal period, because accomplishment of morphogenesis of the compound eye occurs in the last half of the pupal period. As a result, unusual membranous structures such as trophospongium-like structures and myeloid bodies were observed in the cytoplasm of the retinular cells besides the normal rhabdom. This result suggests that FG treatment influences on some steps in the formation of rhabdom membranes. However, the amount of chromophore, 3-hydroxyretinal and the responses to white light recorded by the ERG method from FG-treated specimens were not different from the control animals.