Encarsia formosa Gahan (Hymenoptera: Aphelinidae) is an aphelinid endoparasitoid of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae). Encarsia lycopersici (De Santis) is a parasitoid of the same host, but it is not known if both parasitoids occupy the same niche. In this study, the duration of development of the parasitoids, and the ability of both species to parasitize the 4 preimaginal instars of T. vaporariorum were determined. Results indicate that both E. formosa and E. lycopersici can parasitize all preimaginal instars of T. vaporariorum, and thus effectively occupy the same parasitoid niches, and they are part of the same parasitoid guild. However, variations in parasitism of the different preimaginal instars were observed. Encarsia lycopersici showed a higher parasitism rate on the first instar than on the third and fourth instars of T. vaporariorum. In contrast to E. formosa, which exhibited a lower parasitism rate on the first instar than on the fourth instar. In addition, E. lycopersici showed higher parasitism rate than E. formosa on the first instar of this host. This suggests that E. lycopersici may have a competitive advantage in exploiting the host, as it is the first to occupy it.