The activities of α-NA carboxylesterase, β-NA carboxylesterase, acetylcholinesterase and glutathione S-transferase in laboratory populations of the whitefly Bemisia tabaci biotype B reared on cotton, poinsettia, eggplant and tomato were examined. The ratio of α-NA carboxylesterase activity compared to β-NA carboxylesterase activity in all populations was found to be greater than 1. Further, hydrolyzation to the α-NA form was greater than the β-NA form in all 4 host populations. The frequency of α-NA and β-NA carboxylesterase activity in B.tabaci biotype B also differed between the four populations. The carboxylesterase activity for the tomato population was the highest at 93.06 mOD/(mg protein•min), or 1.49 times greater that for the cotton population. Acetylcholinesterase activity for the eggplant population was significantly (p< (0.01) higher compared to the other 3 host populations. The frequency of acetylcholinesterase activity at >4U/mg protein in the eggplant population was 85%, which was obviously different with the other 3 host populations. The glutathione S-transferase activity of the eggplant population was the highest among the 4 host populations. The difference of glutathione S-transferase activity between populations on eggplant and the other 3 was significant (p<001) at >170U/mg protein. These results indicate that the detoxification enzymes in B.tabaci biotype B reared on different host plants are indicative of physiological plasticity, suggesting an ecophysiological basis for the high degree of polyphagy observed for the biotype B.