This work monitored changes in some digestive enzymes (trypsin and aminopeptidase) associated with the building up of resistance in Cx. pipiens larvae to two chemical insecticides (methomyl and/or malathion) and one biological insecticide
(Bacillus thuringiensis-H14 or B.t H14). The LC50 value of methomyl for both field and the 12th generation (F12) of the
selected strain was 1.789 ppm and 8.925 ppm respectively. The LC50 value of malathion for both field and the F12 of the selected strain was 0.082 ppm and 0.156 ppm respectively, and those of B.t H14 of field strain and the F12 was 2.550ppm & 2.395ppm respectively. The specific activity of trypsin enzyme in control susceptible colony was 20.806±0.452μmol/min/mg protein; but at F4 and F8 for malathion and methomyl treated larvae were 10.810±0.860 & 15.616±0.408 μmol/min/mg protein, respectively. Trypsin activity of F12 in treated larvae with B.t.H14 was 2.097±0.587μmol/min/mg protein. Aminopeptidase specific activity for susceptible control larvae was 173.05±1.3111μmol/min/mg protein. This activity decreased to 145.15±4.12*, 152.497±6.775 & 102.04±3.58a μmol/min/mg protein after larval (F12) treatment with methomyl, malathion and B.t H14 respectively.