In this study, effects of some insecticides (cypermethrin, deltamethrin, dichlorvos, imidacloprid and lambda-cyhalothrin) on glucose 6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ oksidoreductase, EC 1.1.1.49; G6PD) from gilthead sea bream (Sparus aurata) have been investigated in vitro. In this purpose, G6PD enzyme was purified 1864.3-fold with a yield of 54.6% from fish liver using 2’,5’-ADP Sepharose 4B affinity chromatography. Activity of the enzyme was determined by Beutler method spectrophotometrically. The purify of the enzyme was checked by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and the purified enzyme showed a single band on the gel. Additionally, inhibitory effects of some insecticides (cypermethrin, deltamethrin, dichlorvos, imidacloprid and lambda-cyhalothrin) on G6PD were investigated. For this aim, IC50 values for cypermethrin, deltamethrin, dichlorvos, imidacloprid and lambda-cyhalothrin were determined by plotting activity % vs. [I]. IC50 values of cypermethrin, deltamethrin, dichlorvos, imidacloprid and lambda-cyhalothrin were 0.237, 0.016, 0.071, 0.021 and 0.187 mM, respectively. From these results, we showed that deltamethrin is the most potent inhibitor of fish liver G6PD enzyme