BB5, an alkyl phenoxy polyethylene ethanol formulated in acidic buffer, is an adjuvant added usually to various pesticides to potentiate their activities on various biological systems. Addition of 0.2% BB5 resulted in a considerable increase of thiamethoxam toxicity on Bemisia tabaci (Gennadius) nymphs. According to LC50 values, the combined effect was over 25-fold higher than that of thiamethoxam alone. On the other hand, no appreciable combined effect was observed when BB5 and thiamethoxam were applied together on B. tabaci adults. The importance of these results is discussed.