Acidification of the luminal fluid in the epididymis is believed to play an important role in sperm maturation. Previous studies have shown that specialized cells in the epithelium lining the epididymis contain high levels of carbonic anhydrase and that these cells have rod-shaped intramembraneous particles when examined by freeze fracture. Both of these features are characteristic of proton-transporting intercalated cells in the kidney collecting duct. We now show that apical cells in the head of the epididymis and clear cells in the body and tail of the epididymis express high levels of a vacuolar proton-pumping adenosinetriphosphatase on their apical plasma membranes and on intracellular vesicles. By analogy with kidney intercalated cells, these cell types may be specialized for acid secretion in the epididymis.