One susceptible and two resistant clones of the green peach aphid, Myzus persicae (Sulzer) were selected by 10 ppm of fenitrothion for six generations. The three tested aphid clones were: A susceptible clone with normal chromosomal karyotype and red colour (S); a resistant clone with a normal karyotype and red colour (RDR); and a resistant clone with A1,3-autosomal translocation and green colour (GNR). Each clone was started with parthenogenic female. Values of LD₅₀ and slope for two carbamates (carbaryl and pirimicarb) and two organophosphates (fenitrothion and tetrachlorvinphos) were determined topically for the three tested clones before selection. Data revealed that pirimicarb was the most toxic compound against the susceptible clone but tetrachlorvinphos was the least active one against all tested clones. Fenitrothion was the most toxic compound against each of RDR and GNR. The two resistant clones showed resistance ratio (RR) toward all tested compounds compared to the susceptible clone. Values of RR for RDR and GNR clones, respectively, were 30 and 30.8 fold against pirimicarb; 11.77 and 7.27 fold for tetrachlorvinphos; 5.24 and 4.82 fold against carbaryl; and 2.88 and 3.00 fold for fenitrothion. Carboxylesterase kinetic parameters were determined for each clone and each generation using 1-naphthylacetate as a substrate. Values of enzyme kinetics revealed that selection pressure for several generations  resulted in some qualitative modification of the enzyme rather than quantitative change. Changing of the carboxylesterase properties in M. Persicae (Sulzer) as affected by selection with 10 ppm of fenitrothion for only six generations may suggest to stop using anticholinesterase compounds in controlling aphids.