Abstract: Codling moth is the most important pest of apple on world. The codling moth Cydia pomonella L. is controlled mostly with chemical insecticides in Turkey. In this study it was aimed to evaluate C. pomonella populations resistance to thiacloprid which were collected from an apple orchard in Isparta. Resistance was evaluated by bioassay and biochemical methods. Chemical concentrations in 6 different dosages had been prepared. The synergistic activity between thiacloprid and piperonyl butoxide (PBO; monooxygenases inhibitor of cytochrome P450), diethyl maleate (DEM; inhibitor of glutathione-S-tranferase GST), and S,S,S, tributyl phosphorotrithioate (DEF; inhibitor of esterase) was studied. Activities of esterase and GST of codling moth were determined by photometric methods. The substrates 1-naphthyl acetate and 1-chloro-2,4-dinitrobenzene (CDNB) were used for esterase and GST, respectively. Thiacloprid resistance levels in Isparta populations were 14.99-fold compared to a susceptible laboratory population. Application of thiacloprid with synergists DEM, PBO and DEF resulted in 1.53-, 1.51-,1.33- fold synergistic ratios, respectively. While esterase enzyme activity in Isparta populations was higher than SV population (1.55 fold), GST enzyme activities didn’t show a significant difference between Isparta and SV populations.