Enzyme cytochemistry was used to identify vesicles containing acid phosphatase in the midgut digestive cells of partially fed females of the cattle tick Boophilus microplus. The vesicles were elongated or tubular in shape and appeared to be involved with the digestion of host bloodmeal. In mature cells, they were sometimes in close contact with large endosomes, which contained host blood. The vesicles were identified as tubular lysosomes because their morphological and cytochemical characteristics were analogous to similar structures described in mammalian cells. This is the first report of such lysosomes in tick gut cells and suggests some parallels with the intracellular structures involved in the digestion process of mammalian cells. Acid phosphatase in tick gut was also assayed biochemically and was shown to be inhibited with 10 mM sodium fluoride. Cytochemistry showed that this inhibitor blocked activity within the cell and on the lumenal cell membrane.