We observed the histopathological effects of several doses of cyphenothrin. Adult guppies (Lebistes reticulatus) were placed in water containing 34.8, 46.5, 53.5 or 60 μg cyphenothrin/L for 96-h and their gills examined. The most common changes at all cyphenothrin doses were the lifting of the epithelial layer from gill lamellae and some necrosis. Degeneration of secondary lamellae due to edema, the shortening of secondary lamellae, and club-shaped lamellae were also observed. The 96-h LC50 for cyphenothrin to Lebistes reticulatus was 48.7 μg/L.