1. The relationship between the size of a female parasitoid and its fitness is a major element of models of optimal clutch size, of host species selection, of the evolution of siblicide in parasitoids and of some sex allocation models. 2. In estimating this relationship, usually only size-dependent longevity and/or fecundity is considered, neglecting differences in searching efficiency for hosts, although this is an important component of fitness. The relationship between a number of constraints and female size was studied in the parasitoid Aphaereta minuta. Larger females were found to have more eggs available, have larger eggs, live longer and have a higher searching efficiency within patches than small females, but searching efficiency for patches and travel speed were found to be independent of size. 3. The size-dependent constraints were combined into one fitness measure. To this end, the characteristics of a natural habitat were measured and, together with the relationships between female size and these constraints, incorporated into a dynamic programming model in order to calculate the fitness of females of different sizes. Fitness was found to increase linearly with female size. 4. Even from very accurate measures of the size–fitness relationship made under laboratory conditions it remains doubtful whether the relationship holds under field conditions. Three experiments to estimate the relationship between female size and fitness in the field were carried out, all showing an increase in fitness with female size. 5. Comparing the results from the field and the laboratory experiments indicates that fitness increases much more rapidly with increasing female size in the laboratory than under field conditions.
