We make and test predictions about how the number of eggs a female parasitoid is carrying (her «egg load») influences her propensity for'host-feeding', the consumption of host body fluids. We first develop a simple dynamic state variable model that predicts that parasitoids should host-feed only when completely empty of eggs. This result reflects an important assumption of the simple model, namely the absence of a delay between host-feeding and the maturation of eggs. We therefore develop two additional models that incorporate a delay by adding extra «pre-egg» state variables to the simple model. Adding even a short delay makes possible a non-zero threshold egg number at and below which the parasitoid host-feeds. We then compare the results of the models with behavioural observations of host-feeding by Aphytis melinus DeBach (Hymenoptera: Aphelinidae)