The frequency distribution of the non-specific esterase activity using a-naphthyl acetate as substrate in susceptible (S-), field parent and cypermethrin resistant (CYP-R) strains were measured in the homogenate of individual larva (about 260 larvae) from each strain. The activity was calculated as nmol a-naphthyl acetate hydrolyzing/min/larva. The results indicated that the differences in esterase activity among the individual larvae were about 3.5, 7.0 and 15-fold in CYP-R-strain, field parent and susceptible strains, respectively. The mean values of a-naphthyl acetate hydrolyzed by esterases from susceptible, field parent, and cypermethrin resistant strains were 17.66, 61.31 and 216.36 nmol substrate hydrolyzing/min/larva, respectively. Based on the mean value of the esterase activity in resistant strain (216.36), it can be calculate the percentage of resistant genotype in the field parent strain which was up to 9.34%.  The frequency distribution of non-specific esterase activity in individual larvae of the three tested strain revealed that the majority of the individuals exhibited a distinguishable activity pattern, consistent with the susceptibility to the pyrethroid cypermethrin. The results suggest that the metabolic detoxification processes via the non-specific esterases play an important role as resistance mechanism against cypermethrin insecticide in the larvae of C. pipiens mosquito.