In a laboratory colony of the asexual solitary endoparasitoid Venturia canescens Grav. (Hymenoptera: Ichneumonidae), two genetically and phenotypically distinct lines (RP and RM) appear to coexist sympatrically, with the reproductive success of the RM-line being greater under con-specific superparasitism but lower under self superparasitism. To investigate the basis of the RM-line's advantage under con-specific superparasitism, we examined the outcome of inter-larval physical combat under in-vitro conditions. The results showed that the outcome depended on both the relative and absolute ages of the contestants, and that the competitive abilities of the two lines were not symmetric. In contests involving a larva and an egg, the unhatched wasp lost. In contests involving two larvae, at least one of which was newly hatched, the RP-larva tended to lose. Finally, if both larvae were at least 8[long dash]10 h post-hatching when the contest occurred, then the larger larva tended to lose, with the probability increasing with the difference in length between the two larvae. Thus, the higher reproductive success of the RM-line under competing superparasitism with the RP-line is due to a physiological difference between the newly hatched larvae of the two lines, which results in an advantage to the RM-larva independent of the order or time interval between ovipositions. Ephestia kuehniella (Pyralidae) : Host Venturia canescens (Ichneumonidae) : Parasite Biometrics; Behaviour; Agonistic behaviour; Aggressive behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Competition Ephestia kuehniella; Hymenopteran parasites; Outcome of parasite interlarval contests; influence of parasite relative & absolute ages; Venturia canescens Venturia canescens; Age; Fighting; Interlarval contests; Lepidopteran hosts; Outcome of interlarval contests; influence of relative & absolute ages; Ephestia kuehniella; Intraspecific competition none