To determine sensitivity to fenitrothion (MEP), fenobucarb (BPMC) and imidacloprid (IMI) in each larval instar of Cheumatopsyche brevilineata, we carried out 48-hr acute toxicity tests using strains M and K originating from an urban river and an irrigation canal, respectively. First instar was the most sensitive stage to all three insecticides whereas fifth instar was the most insensitive in both strains. Larvae of strain K were significantly more insensitive to MEP through larval growth and to BPMC in second or later instars than larvae of strain M; however, sensitivity to IMI did not differ significantly between strains.