1. Many studies examining the relationship between host size, an index of host quality, and parasitoid fitness use development time and/or adult parasitoid size as currencies of fitness, while ignoring pre-adult mortality. Because the physiological suitability of the host may vary in different stages, sizes, or ages of hosts, a misleading picture of host quality may therefore be obtained in cases where fitness is based on only one or two developmental traits. 2. The development of the solitary koinobiont endoparasitoid Microplitis demolitor is examined in different larval age-classes of its host the soybean looper Pseudoplusia includens. Hosts were parasitised on days 1-8 after hatching from the egg, and development time, adult body size, and mortality of the parasitoid were compared. 3. A comparison of larval growth trajectories (using dry body mass) of M. demolitor revealed that parasitoid larvae attained over twice as much body mass in old hosts than in younger hosts. Similarly, adult parasitoid size at eclosion generally increased with host size, although parasitoids developing in smaller hosts lost a much lower proportion of mass between pupation and eclosion. 4. Overall egg-to-adult development was most rapid in intermediate-aged hosts, and longer in hosts at opposite ends of the age continuum. Moreover, parasitoid mortality varied non-linearly with host stage, and was generally higher in very young and older hosts. 5. Based on these results and other empirical data for koinobionts, it is argued that fitness functions in this group of parasitoids are not simply a positive function of host size or age, but instead may be distinctly dome-shaped, both patterns reflecting the degree of physiological and nutritional compatibility between the two organisms. Biometrics; Life cycle and development; Evolution; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics Pseudoplusia includens (Noctuidae); Age; Size; Hymenopteran parasites; Microplitis demolitor; Parasite development, size at eclosion & mortality, host age & size relationships Microplitis demolitor (Braconidae); Size; Development; Natural selection; Fitness measures; Lepidopteran hosts; Pseudoplusia includens; Parasite development, size & mortality in relation to host age & size; Mortality none