Insecticide resistance is usually associated with fitness costs, but such costs may be mitigated by increased energy and amino acid accumulation and mobilization as has been suggested in the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae). To address this adaptation, cysteine proteinases (E.C. 3.4.22), one of the main proteinases in weevils, was purified from an insecticide-susceptible and two insecticide-resistant strains of the maize weevil (one with fitness costs, referred as resistant-cost, and the other without it, referred to as resistant no-cost) using thiol-sepharose affinity chromatography. Purification of the cysteine proteinases revealed a single 74,000 Da molecular mass band in the susceptible strain, two bands of 72,000 and 83,000 Da in the resistant cost strain, and two bands of 68,000 and 74,000 Da in the resistant no-cost strain. Purified cysteine proteinases of the three strains behaved differently regarding casein degradation and inhibition; the proteinases least sensitive to inhibition by the specific cysteine proteinase inhibitor E-64 were those from the resistant no-cost strain as indicated by their highest I(50) value. The pH and temperature profile of cysteine proteinase activity differed among strains and although substrate affinity (i.e. K(M)) of the cysteine proteinases was similar, the V(max) value for cysteine-proteinases from the resistant cost strain was 3-fold and 5-fold higher than V(max) values for the resistant no-cost and susceptible strains respectively. Cysteine proteinase activity was highest for the resistant cost strain rather than the resistant no-cost. Therefore enhanced cysteine proteinase activity is unlikely to be playing significant role in mitigating the costs usually associated with insecticide resistance.