The course of acrosomal differentiation observed during spermiogenesis in two starfishes shows that the central components of the mature acrosome are produced by Golgi activity. In the early spermatid, small Golgi‐derived vesicles enter the hydrated acrosomal mass and appear to contribute their membrane constituents to the acrosomal‐membrane precursor elements. A single lamella of smooth endoplasmic reticulum and fine‐fibrillar material associated with it surround the membraneprecursor complex. In a drastic reorganization by which the spermatid acquires antero‐posterior symmetry, the acrosome becomes embedded in the anterior part of the nucleus directly beneath the plasma membrane. All the other organelles congregate in the posterior cytoplasm; a thin layer of cytoplasm persists around the sides of the nucleus. During late spermiogenesis two additional acrosomal components become increasingly conspicuous. One is the layer of fine‐fibrillar material associated with the smooth endoplasmic reticular vesicles surrounding the Golgi‐derived elements. This material is finally pushed towards the center of the sperm head by a late accretion of fibrous product which appears to be synthesized throughout spermiogenesis by the ribosomes, and accumulates around the anterior part of the acrosome as the cytoplasmic matrix diminishes.