In an effort to explain different reproductive strategies among parasitoid wasps, differences in potential fecundity of members of the family Ichneumonidae were compared in relation to the availability of the host and the probability of survival of the parasitoid once it was associated with the host. As host abundance declines during a generation, the ovarioles per ovary are less numerous in parasitoid species attacking successively less abundant stages of the host. Those parasitoids that attack well-concealed hosts relative to their body size (inferred from a high ovipositor length-body length ratio in the parasitoid) have fewer ovarioles than those attacking exposed hosts. A decline in storage capacity for mature oocytes both in the ovarioles and in the oviduct, and an increase in the size of eggs produced, is associated with a decline in ovariole number.