Many phytophagous insects are highly specific with respect to the plants on which they lay eggs, restricting oviposition to plant species within one family or group of related families (Brower and Brower, 1964; Ehrlich and Raven, 1964; Eastop, 1973), or even to species or groups of species within a particular family or genus (Singer, 1971; Janzen, 1974; Wiklund, 1974, 1975; Benson et al., 1976; Chew, 1977; Ives 1978; Smiley, 1978). Although the proximate cues used for host selection by ovipositing females are known for some highly specific species (Gupta and Thorsteinson, 1960; Singer, 1971; Staedler, 1974; Nishida, 1977; Ichinose and Honda, 1978; Rausher, 1978), little is known about the advantages associated with preferring to lay eggs on some species of plants rather than on others. In this study I evaluate two alternative hypotheses that account for oviposition preference in a butterfly. Females of the pipevine swallowtail butterfly, Battus philenor, search preferentially at any one time for only one of their two host plants in east Texas (Rausher, 1978). One hypothesis that can explain this search preference is that it ensures that females lay most of their eggs on the plant species on which eggs and larvae have a greater probability of surviving to the adult stage (Chew, 1975, 1977; Feeny, 1975; Gilbert and Singer, 1975; Wiklund, 1975; Smiley, 1978). In some situations, however, the host species on which juvenile survivorship is greater may be rarer than other potential host species. Under such circumstances, the advantage gained by preferentially searching for and ovipositing on the plant species on which juvenile survivorship is greater may be more than offset by the disadvantages associated with searching for the rarer host species. If females have a short life expectancy, for example, they may discover fewer plants and lay fewer eggs before dying if they search preferentially for the rarer host species than if they search preferentially for the more abundant host. Consequently, a second hypothesis that accounts for a specific search preference in ovipositing B. philenor females is that it ensures they search for and lay most of their eggs on the more abundant plant species. Although these two alternatives are not necessarily mutually exclusive, it has been possible to distinguish between them in this study.