An aspartic endopeptidase named THAP, from the eggs of the tick Riphicephalus (Boophilus) microplus, has been suggested to be involved in vitellin-degradation. Here we characterized this enzyme further, showing that THAP mRNA is present in the fat body, midgut and ovary of ticks, in two developmental stages (partially and fully engorged females). However, higher transcription levels were found in fully engorged vitellogenic females. The THAP protein was detected in the haemolymph, midgut and fat body and, in higher quantity, in the ovary of fully engorged females, and it was present throughout embryo development. The protein is synthesized as a higher molecular mass form and after the onset of embryogenesis THAP is converted into an active form by autocatalysis. We also produced a recombinant protein (rTHAP) in E. coli that was active in the fluorogenic peptide substrate and able to hydrolyze vitellin from 7-day-old eggs in a reaction that is heme-sensitive and inhibited by pepstatin A. However, rTHAP does not hydrolyze vitellin from 1 and 12-day-old eggs. As a result, we suggest a model for THAP synthesis, transport, storage and activation and for the role it plays in embryonic development by participating in vitellin processing.