In order to understand fine structural and histochemical changes in the cyst cell and he interstitial cell in the testis of the spottybelly greenling Hexagrammos agrammus associated with the reproductive cycle from January to December, 1992, there cells were studied by electron microscopy and light microscopy. The cyst cells in the mature testis show a weak affinity to haematoxylin. while they become larger in size. At this time, these cells appear to be more functional than those on the growing stage because several mitochondria, endoplasmic reticulum, glycogen particles and a few lipid droplets appear in the cytoplasm of the cyst cell. It appears, therefore, that the cyst cell of this species has vital functions for nutrition, secretion and steroidogenesis. Well-developed interstitial cells contain large rod-shape or spherical mitochondria with tubular cristae and the large quantities of smooth endoplasmic recticulum and electron-dense materials in the vesicle at the mature and spawning stage. The interstitial cells of this species show characteristics of steroid interstitial cells having a vesicular nucleus, mitochondria with tubular cristae, and smooth endoplasmic reticulum. However, these interstitial cells of teleost give negative histochemical reactions for Sudan black B.