The rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae) was collected from El-Behera governorate (EB). The
first generation showed reduced susceptibility to deltamethrin and malathion compared with the laboratory susceptible
strain (LS). Susceptibility test results (using LS colony as a reference strain) indicated that EB strain has relative toxicity
ratio, RR50 and RR90 of 16.64 and 9.08 for malathion. Although, EB strain was tolerant to deltamethrin with RR50 of 5.2, it
was marginally resistant to cypermethrin and entirely susceptible to permethrin. The biochemical results showed elevation
in the activity of total esterase (ES), carboxylesterase (CE) and glutathione S- transferase (GST) which suggested that the
metabolic resistance may has a key function in this population. Furthermore, acetylycholinesterase (AChE) activity in EB
strain was 3.52 times higher compared with LS strain which pointed the insensitivity of AChE in EB strain. Moreover,
esterase analysis indicated genetic polymorphism between EB and LS populations which might be attributed to selection
pressure due to continuous exposure to insecticides. This study suggested the existence of malathion and deltamethrin
resistance in EB strain which might be a consequence of biochemical alterations. The results of current study indicate the
importance of continuous monitoring of resistance of stored product insect pests to plan successful management strategies.