The rates of establishment of exotic natural enemies introduced against exotic pests in the orders Homoptera, Lepidoptera, and Coleoptera were found to be inversely related to (1) the number of species released at a given time and place and (2) the number of exotic incumbent species of natural enemies present. Thus, competitive exclusion of introduced natural enemies has probably occurred and contnbuted to the relatively low rate of establishment in biological control. It is suggested that, in cases in which less-than-complete control was obtamed through multiple-species releases, natural enemies capable of effective control of the target pest may have been competitively excluded. Because of this possibility, use of the empirical approach of releasing all available species of natural enemies, with the hope that the best species or combination of species will be sorted out in the field, should be questioned. In modern biological control, a more rational release strategy is in order.