Viability and oviposition of the parasitic wasp Nasonia vitripennis on its host Phaenicia sericata at various parasite—host densities were analyzed. As the parasite density becomes excessive, an increasing portion of the progeny is reduced. These density—dependent reduction mechanisms are: retention of eggs if a sufficient number of hosts is not available; death and decomposition of superparasitized host pupae resulting in death of the parasite larvae; food shortage within the host leading to parasite mortality shorty before and during metamorphosis; reduced longevity and fecundity of viable female offspring from superparasitized hosts and a decreasing portion of female progeny as the parasite number on a given host supply increases. This change in sex ratio is relatively independent of the number of eggs laid per parasite female or per host pupa. It is mainly determined by the number of simultaneously ovipositing females: the more dense the ovipositing females, the less fertilized eggs they produce. Two categories of density—dependent regulation mechanisms are distinguished: behavioural and pathological mechanisms. The significance of the two categories for the existence of a theoretical population equilibrium on the one hand and for a real equilibrium on the other hand is discussed.