The extensive use of insecticides to control insect pests has provided a dramatic illustration of evolution in natural populations. Malathion resistance in the red flour beetle, Tribolium castaneum (Herbst), is a worldwide problem and is very stable once it becomes widespread in natural populations. It is not surprising that resistant and susceptible 5 strains differ in properties other than their adaptation to insecticides, such as development time, fecundity and fertility. The present study investigated the biochemical mechanisms of resistance in a highly malathion specific resistant strain of T. castaneum (PH-1) which was kept under repeated selection pressure resulting in a very high level of resistance as compared to the susceptible strain, and its fitness against oxidative and pathogenic stresses. General esterase, malathion-specific carboxylesterase, glutathione S-transferase and target site sensitivity were compared, in this and a susceptible strain, to better understand biochemical mechanisms of resistance.