BACKGROUND Many bat species have an extra set of major salivary glands. In some species, the accessory glands are quite similar to the principal one, but in others they may be radically different. Accessory glands usually are associated with the submandibular gland, but the free-tailed bat, Tadarida thersites, also has an accessory parotid gland. In the present study, we compared the accessory parotid gland with its principal counterpart. METHODS Salivary glands were removed from two specimens of free-tailed bats, one of each sex, that had been live-trapped in western Kenya and immersion-fixed in a specially formulated mixture designed for field fixation. Once back in the laboratory, the tissue was further prepared for electron microscopy by conventional means. RESULTS The secretory endpieces consist of serous tubules composed of typical serous cells that contain numerous dense granules. In contrast, the intralobular duct system shows a radical departure from normal. These ducts are enormous in caliber, their lumina measuring greater than 100 microm in diameter. They appear to arise by amalgamation of the homologues of intercalated and striated ducts into macroducts. The walls of the macroducts consist of intermingled patches of simple cuboidal and simple columnar epithelia that occasionally include a tuft cell and are underlaid by an almost continuous layer of myoepithelium. A few cells have some modified basal striations, but most cells display a cytological organization that differs radically from either of their two putative ancestral cell types. Both tall and short epithelial macroduct cells have a paranuclear collection of ovate mitochondria and aggregates of what presumably are peroxisomes. Macroduct cells in both the female and male are pervaded by a system of tubular smooth endoplasmic reticulum (SER). In the female, the SER gives rise to membranous whorls that consist of numerous plies. As the macroducts approach their termini, a single row of small dense secretory granules appears just beneath their luminal surface. At the lobular periphery, the ducts taper down to become excretory ducts of normal dimensions. CONCLUSIONS An accessory parotid gland occurs in T. thersites, but apparently is absent in the related species, T. brasiliensis. The ultrastructural data are consistent with a possible steroidogenic function, although other features of the gland might relate to the elaboration of a secretory product associated with feeding on chitinous beetles. The macroducts conceivably function as reservoirs of preformed saliva.