Esterases are often used to measure genetic variation, yet they may be influenced by external factors. Isoelectrofocusing was used to investigate the effects of bloodmeal on the esterase variation in wild populations of Phlebotomus (Larroussius) ariasi Tonnoir and P. (L.) perniciosus Newstead and in a colonized population of P. (Phlebotomus) duboscqi Neveu-Lemaire. Modified electrophoretic profiles were demonstrated to be associated with blood digestion. The bloodmeal-induced esterase electromorphs were only found in the abdomen homogenates of the sand flies. These changes in esterase phenotype were probably due to a proteolysis phenomenon and disappeared at the end of bloodmeal digestion. The effect was eliminated by adding trypsin inhibitors.