Two types of exocrine rosette glands (type A and type B), located in the gill axes of the grass shrimp, Palaemonetes pugio, are described. The type A glands are embedded within the longitudinal median spetum of the gill axes, whereas the type B glands typically project into the efferent hemolymph channels of the gill axes. Although both glands have certain common characteristics (i.e., a variable number of radially arranged secretory cells, a central intercalary cell, and a canal cell that forms the cuticular ductule leading to the brachial surface), they differ in the following respects. The type B gland is innervated, but type A is not; axonal processes, containing both granular (ca. 900-1300 A) and agranular (ca. 450-640 A) vesicles, occur at a juncture between adjacent secretory cells and the central cell of the type B gland. The secretory cells of type A and B differ in their synthetic potential and membrane glands, most frequently encountered in larger (24-28 mm, total length) grass shrimp, than in the underdeveloped, immature glands that are most abundant in smaller (14-18 mm, total length) grass shrimp. Thus, in mature glands, the secretory cells of type A are characterized by extensive RER, abundant Golgi, and numerous, secretory granules, whereas the secretory cells of type B are characterized by extensively infolded and interdigitated basal plasmalemmas and by the presence of numerous mitochondria.