The juxtaglomerular (JG) cells were demonstrated in the carp kidney by stains of Bowie, periodic acid Schiff-hematoxylin and toluidine blue (pH 12). Ultrastructurally, the JG cells had a large electron-lucent nucleus, a well-developed Golgi apparatus, a rough surfaced endoplasmic reticulum, cisternae of which were occasionally enlarged, minute secretory granules of about 230nm in diameter and contractory elements consisting of myofilaments and attachment bodies. By immunocytochemistry using anti-renin serum, numerous immunoreactive JG cells were demonstrated mainly in the areas of the tunica media of the arterioles or the small arteries, and occasionally in the boundary region between the tunica media and the tunica adventitia of the arterial vessels. In each cell, the immuno-reactivities were polarized toward the adventitial side rather than the medial side. Renin-immunoreactive cells were observed neither in the walls of the efferent arterioles nor in the mesangial regions. These results suggest that a definite control system of blood pressure via the renin-angiotensin series functions in the mesonephros in fishes as it does in the metanephros in mammals.