The ultrastructure of the parotid gland was examined in two purebred strains of dogs, as well as in mongrels. The seromucous acinar cells contain numerous secretory granules of unusual substructure that varies somewhat from one granule to another. The most common configuration consists of a large, dense spherule surrounded by a thin electron-lucent halo. The matrix is composed of a series of dense beads, each delimited by a light ring. Between these beads is a material of intermediate density that sometimes forms a corona around the spherule. The organization of the granule is somewhat altered when osmication is omitted from the fixation procedure. Striated ducts have basal regions that are of conventional appearance, but the cell apex is characterized by a collection of ellipsoidal structures oriented perpendicular to the luminal plasma membrane. At least in some cases these are joined to the plasma membrane by fine threads. Despite the obvious phenotypic differences in the canine breeds used in this study, their parotid glands were identical, indicating that the genetic mechanisms that control gland structure, especially secretory granule substructure, have not been affected by selective breeding.