1. In multiparasitism of larvae of the moth Ephestia serecarium by the solitary ichneumonid endoparasitoids Horogenes chrysostictos and Nemeritis canescens, neither has an intrinsic superiority over the other and free competition occurs between their immature stages for possession of the host.
2. There is no interaction between the eggs of these species before they hatch.
3. When larvae of these two species are present in the same host at the same time they compete for possession of the host by one of two mechanisms, physical attack or physiological suppression.
4. When the larvae hatch from the eggs at the same time, neither has a clear advantage over the other and either species may win by physically attacking the other with its mandibles.
5. If they are of different ages, the older larva usually wins by physical attack when the difference in age is less than 40 hr. at 25°C.
6. If the difference in age is 50 hr. or more at 25°C. the older larva always wins in competition by physiological suppression of the younger one.
7. The factors which are important in determining the result of competition are: difference in age between the ist-instar larvae at eclosion, environmental temperature and the species of host attacked, which determine the parasites' rate of development.
8. A hypothesis of asphyxiation is presented with experimental evidence of its validity as an explanation of the physiological suppression of supernumerary parasitoids. It is suggested that asphyxiation by oxygen-lack can better account for physiological suppression than the theories of toxic secretion, specific inhibitor and starvation.