Abstract: Many species of parasitoids not only parasitize whitefly hosts but can also feed them. Host feeding and parasitism preference in Eretmocerus hayati for nymphs of Bemisia tabaci Q biotype and Trialeurodes vaporariorum was studied under no-choice and paired-choice conditions. In no-choice test, the number of 1st to 4th instar nymphs of B. tabaci predated by a female E. hayati in 48 h was 10.7, 6.4, 6.7, and 5.0, respectively, and the number of 1st to 4th instar nymphs parasitized was 22.1, 27.5, 25.4, and 16.5, respectively. In contrast, the number of 1st to 4th instar nymphs of T. vaporariorum fed by a female E. hayati in 48 h was 4.4, 3.7, 2.8, and 2.3, respectively, and no nymphs was parasitized. In paired-choice test with the same stage nymphs of B. tabaci and T. vaporariorum, the number of 1st to 4th nymphs of B. tabaci fed by E. hayati was 1.8, 2.6, 2.5, and 1.5 folds more than those of T. vaporariorum, respectively. These results indicate that whitefly host species and host stage affect host feeding and parasitism capacity of parasitoids. E. hayati has the potential to be used as a bio-control agent for control of the invasive whitefly B. tabaci, but not for the indigenous whitefly T. vaporariorum.