The B cells of dog pancreatic islets were studied by electron microscopy. The crystalloid core of secretory granules in B cells is unique. The crystallization of the secretory granule core appears to be accomplished in the Golgi apparatus. The square or hexagonal network with a periodicity of 30–60 Å that is produced suggests that the unit cell is either hexagonal or rhombohedral. The electron opaque dots which constitute the periodic densities may represent the insulin molecule itself in a crystalloid array.