Abstract: The histochemistry and the ultrastructure of STH cells (somatotropin cells) of the adenohypophysis in Ctenopharyngodon idellus have been studied.STH cells lie in the central region and the back region of the meso-adenohypophysis.The results showed that STH cells are acidophilic cells,orange G positive and PAS-AB negative during both PMB (PAS-MB) dyeing and APG (AB-PAS-OG) dyeing;STH cells show high electron density,their rough-surfaced endoplasmic reticulum in general appears as well-developed parallel lamellae around the nucleus,STH cells have many granules in small size under the electron microscope.