The pupal parasitoids Coccygomimus turionellae L. and Herpestomus brunneicornis Grav. are characteristic members of the parasitoid complexes associated with Yponomeuta spp. Both species suffer high mortalities by hyperparasitism and indiscriminate host feeding. In two subsequent years the degree of hyperparasitism was density dependent. This suggests that hyperparasitoids may stabilize the populations of pupal parasitoids of Yponomeuta . In C. turionellae oviposition follows a Poisson distribution, in H. brunneicornis it is aggregated and can be described by Taylor's "Power Law". These differences in oviposition behaviour are discussed in connection with differences in the host range and ovipositor size in C. turionellae and H. brunneicornis .