Monoctonus paulensis; SIZE-; Hind tibia length, mass scaling; WEIGHT-; Body mass, size allometry, effects of hemipteran host species & stage; GROWTH-; Allometry, body size & mass relationships; NATURAL-SELECTION; Fitness, body mass & size scaling relationships; Effects of hemipteran host species & developmental stage, laboratory study; HEMIPTERAN-HOSTS; Aphidoidea; Parasitoid body mass & size scaling, effects of host species & stage Acyrthosiphon pisum; Macrosiphum creelii; Myzus persicae; Sitobion avenae; HYMENOPTERAN-PARASITES; Monoctonus paulensis; Parasitoid body mass & size scaling, effects of host species & stage APHIDOIDEA-; PHYSIOLOGICAL-CONDITION; Quality, effects on hymenopteran parasite body mass & size allometry; NYMPHAL-DEVELOPMENT; Second & fourth stage, effects on hymenopteran parasite body mass & size; GROWTH-; Potential, hymenopteran parasitoid fitness relations, allometric evidence; HYMENOPTERAN-PARASITES; Monoctonus paulensis; Parasitoid body mass & size scaling, effects of host species & stage We describe the allometry of body mass and body size as measured by hind-tibia length in males of Monoctonus paulensis (Ashmead) (Hymenoptera: Braconidae, Aphidiinae), a solitary parasitoid of aphids. To assess the influence of host quality on allometric relationships, we reared parasitoids on second and fourth nymphal instars of four different aphid species. Acyrthosiphon pisum (Harris), Macrosiphum creelii Davis, Myzus persicae (Sulzer) and Sitobion avenae (F.), under controlled conditions in the laboratory. Dry mass was positively correlated with hind-tibia length, and could be predicted from it, in unparasitized aphids, in aphid mummies containing parasitoid pupae, and in the parasitoid. The reduced-major-axis scaling exponents for the regression of dry mass on hind-tibia length were species-specific in aphids, reflecting differences in volume and shape between species. In mummified aphids, the stage at death influenced the size/mass relationship. In males of M. paulensis, the allometric exponent varied between parasitoids developing in different kinds of host. Individuals developing in pea aphid were absolutely larger in dry mass as well as proportionately larger relative to their hind-tibia length. We discuss the allometry of body size and body mass in relation to parasitoid fitness.