Abstract: The parasitism preference of Encarsia formosa Gahan to different nymphal instars of the whitefly Bemisia tabaci (Gennadius) on tomato was studied at 25℃ in laboratory. The development, survival, adult longevity and fecundity of the parasite were observed under constant temperatures. The results showed that E. formosa parasized all the four instars of the whitefly, but preferred the 3rd and pre-4th instars. When different instars of the host were exposed simultaneously to this parasite, the developmental period of E. formosa 3rd and 4th instars was significantly shorter than that of the 1st and the 2nd instars. The developmental period of the parasite from egg to adult and the longevity of the female wasp decreased with the increasing temperature. The juvenile survival rate of E. formosa was the highest at 25℃ and the lowest at 15℃, while different temperatures had no distinct difference on the fecundity of the wasp.