Abstract: Resistance mechanisms to pyrethroids were investigated in Bemisia tabaci （Gennadius）with synergist bioassay, biochemical analysis and RT-PCR amplification for ⅡS4-6 region of sodium channel gene. Significant synergisms by PBO and TPP but no obvious synergism by DEM to cypermethrin, deltamethrin, lambda-cyhalothrin and fenpropathrin were found in field-collected resistant strains of B. tabaci. The carboxylesterase （CarE） activities to α-naphthylacetate and β-naphthylacetate in the resistant strains were 2.16－2.65 and 1.22－1.41 times that in the susceptible strain respectively, but there was no difference in the GST activities among the resistant strains and the susceptible strain. This indicated that CarE and mixed function oxidase （MFO） played an important role in the resistance of B. tabaci to pyrethroids, while the resistance of B. tabaci to pyrethroids was not related to GSTs. Nucleotide and deduced amino acid sequences of ⅡS4-6 region of sodium channel in six different field strains of B. tabaci in Fujian province was amplified by reverse transcriptase-mediated polymerase chain reaction analysis （RT-PCR）. Two mutations were identified in theⅡS4-6 linker of the para-type sodium channel of B. tabaci： isoleucine to

Key words: Bemisia tabaci, insecticide resistance, pyrethroids, resistance mechanisms