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1. Aphaereta minuta (Hymenoptera: Braconidae) is a gregarious larval-pupal endoparasitoid of many Diptera species. Several larval instars can be parasitized and the size differences between host species can be considerable. After parasitization, however, the host larva continues to grow and the parasitoid's eggs hatch after the host pupates. We question whether this delay between the moment of oviposition and that of resource availability for offspring development hampered the ovipositing female in making optimal clutch size decisions. 2. Using an optimality approach we analysed the relationship between clutch size (number of eggs) and fitness in different instars of the host Delia antiqua in the laboratory. 3. Clutch size was artificially manipulated, and the relationship between clutch size and fitness was quantified using the following parameters: offspring egg to adult survival, sex ratio of emerging adults and size of emerging daughters (since size and number of eggs are positively correlated, size is a measure of fecundity). 4. Survival to adult stage was slightly positively correlated with clutch size in first instar host larvae and negatively correlated with clutch size in second instar host larvae. Sex ratio (proportion daughters) increased with increasing clutch size. The size of both males and females on emergence was negatively related to clutch size, and more strongly to the number of emerging adults. 5. The calculated Lack clutch size (whereby fitness is maximized per clutch) increased with larval host stage, as did the observed clutch size. 6. For each instar the observed clutch size was lower than the calculated Lack clutch size. We argue that under natural conditions females are selected to lay a clutch lower than the Lack clutch size.