A female Pepsis cupripennis attacked females and large juveniles of Acanthoscurria suina and Eupalaestrus weijenberghi in their own burrows, under laboratory conditions. Males of both spider species were also attacked in open arenas but with minor success. The wasp buried the immobilized tarantulas in special chambers excavated from the spider's burrows. An egg was deposited by the wasp on the spider's abdomen and its development into an adult wasp was studied. On two spiders the eggs were spoilt and the hosts completely recovered. In the other six spiders development proceeded, reaching the adult stage in five cases. Two female wasps were obtained from large spiders and three males were obtained from small hosts. The wasp apparently needed a spider six times heavier than herself to obtain a daughter. Spider size estimation by the wasp and ecological defensive mechanisms of the spiders are discussed.