The unicellular hatching glands (UHGs) of four species of salmonid fish, Salmo gairdneri, Salmo trutta, Salvelinus fontinalis and Salvelinus pluvius were studied by light and electron microscopy. The UHGs are distributed on the epidermis of head and yolk sac, and on the epithelium of the mouth and gills. Since these cells are large and include dilated cisternae of the endoplasmic reticulum from the primitive to the mature stages they are conspicuous. Around the Golgi complexes, there are consecutive figures showing synthesis of secretory granules with close relationships to the Golgi vesicles. The secretory granules grow in size and vary in density during maturation; some have enclosed cytoplasmic structures. At the hatching stage, they are discharged with some cytoplasmic structures from the UHG which is located in the superficial layer of epithelium. After exhaustion of the secretory granules, the remainder of the contents of UHGs is liberated.