In theory, a parasite whose effective reproductive rate remains, under practically all conditions, equal to, or greater than, the effective reproductive rate of its host, ought eventually to annihilate it, or, at least, to produce an abrupt reduction in the host population, so great, that it is almost impossible for the vast majority of the emerging parasites to find any prey; so that the parasite population suffers, in its turn, an abrupt and catastrophic reduction, after which the host again increases, followed in due course by the parasite. The numerical fluctuations occurring as a result of this interaction, under various conditions, have been the subject of much theoretical study, but relatively little is known about what really happens in nature. The following brief account of the development of a colony of the Chalcid parasite, Aphelinus mali Hald., at the expense of its host, Eriosoma lanigerum Hausm., may therefore be of interest.