The ecto-5'-nucleotidase from the cattle tick Boophilus microplus is an unusual enzyme, hydrolysing a variety of nucleoside mono-, di- and triphosphates to release the free nucleoside. The gene has been sequenced and the recombinant protein expressed as a functional, active enzyme. Nevertheless, the function of the enzyme in the tick remains obscure. The enzyme is present throughout the life cycle, but in largest amounts in unfed larvae and adult ticks. The tissue location has been studied in adult female ticks by Western blotting, RT-PCR and immunofluorescence. All methods show the enzyme to be principally in the Malpighian tubules, though significant amounts are also present on the surface of ovaries and in detectable amounts in other tissues. This, together with the known specificity of the enzyme, suggests a role in purine salvage pathways. Sensitivity of ticks to allopurinol, an inhibitor of hypoxanthine-guanine-phosphoribosyltransferase, supports the importance of purine salvage in this tick and the potential role of nucleotidase in this pathway.