In many host-parasite relations, the parasitic species has numerous variants, antigenic strains, or types. The host species also has many types of reactions and defenses, such as specific antibodies, cellular defense mechanisms, and genetically determined resistances or susceptibilities. We propose here a simple phenomenological model, based on a game with random payoffs, to explain how the evolution by natural selection of antagonistic hosts and parasites may lead to large and roughly comparable numbers of strategies of parasitic attack and host defense. Diversification in strategies of parasitic attack and host defense may lead to, be accompanied by, or be a consequence of speciation. Examples of parasitemediated speciation were reviewed by Price et al. (1986, pp. 498-499). Because the model we propose relates to interspecific diversity as well as to intraspecific diversity, the model provides a theoretical basis for Eichler's rule. Eichler's rule asserts that "when a large taxonomic group, for example, family, of hosts consisting of many species is compared with an equivalent taxonomic group consisting of few representatives, the large group has the greater diversity of parasitic fauna" (Noble and Noble 1982, p. 461). In support of Eichler's rule, Price reviewed "many examples ... of a significant relationship between number of species in each host category and number of [species of] parasites that exploit members of that category" (1980, pp. 27-29). After we describe the game-theoretical model and analyze its properties mathematically, we interpret it in greater detail. We describe what the model achieves and what the model omits or ignores. We consider other explanations of the diversity of host and parasitic adaptations and review some other uses of game theory to describe host-parasite relations. Kinds of Models of Host-Parasite Coevolution Models of host-parasite coevolution fall into three broad classes (Holmes 1983, pp. 176-177). In mutual-aggression models, selection acting on the parasite diametrically opposes selection acting on the host. The parasite is selected to