In this paper we address two questions concerning the interaction between fleshy-fruited plants and their seed dispersers: (1) What determines optimal disperser coterie size (designated as Ĉ) and (2) Why does disperser specialization occur along taxonomic lines? We review factors that affect the evolution of Ĉ and conclude that seed size and disperser quality (designated as Q) are especially important. We present a simple graphical model for determining Ĉ based on Q and conclude that Ĉ will be small (i.e. a specialized disperser strategy) when Q declines rapidly with increasing coterie size; Ĉ will be large when Q declines slowly with increasing coterie size. We construct a model based on fitness set theory to predict that specialization on particular vertebrate taxa (e.g. birds or mammals) will be favored when different disperser taxa are perfectly substitutable or antagonistic (sensu Tilman 1982); mixed-taxa coteries are favored when different disperser taxa are complementary. Finally, we predict that when conditions favor taxonomic specialization, plants will evolve bird fruits more often than bat or primate fruits because of the greater species richness of birds compared with bats and primates. © 1993 Kluwer Academic Publishers.