After oviposition the uniparental hyperparasite Signiphora coquilletti Ashmead was observed to spin a web over her host, Trialeurodes vaporariorum (Westwood) previously parasitized by Encarsia formosa Gahan. Signiphora coquilletti was subsequently reared from webbed T. vaporariorum pupae and from webbed pupae of Tetraleurodes mori (Quaintance), T. stanfordi (Bemis), Aleuroplatus coronatus (Quaintance), and A. gelatinosus (Cockerell). Oviposition and web-spinning behavior of S. coquilletti are described, and the ultrastructure of the webs is discussed. We believe that webs may function as a physical barrier to host searching females of competing Encarsia species, or that they may serve as an intraspecific host-marking device. Two additional hypotheses are that webs afford protection against predation of Signiphora immatures in host pupae, or that they reduce mortality of Signiphora by tying host pupae to leaf surfaces.