Physical components of host choice were examined in two Rhagoletis species that have recently become pests of sour cherry (Prunus cerasus L.) in Utah: R. indifferens Curran, which is widespread on cherry and apparently has no native host, and R. pomonella (Walsh), which is widespread on native hawthorn and infrequently attacks cherry. When presented with artificial hosts, the two species responded similarly to host size but not to host color. Color preferences of R. indifferens females were orange > black > red, while those of R. pomonella females were red > orange > black. Size preferences were measured either as the frequency of attempted oviposition into different-sized spheres or as the number of eggs laid in different-sized wax domes. Results from the two kinds of assays were similar if egg densities in domes were expressed in terms of available surface area. Each fly species generally failed to discriminate between small (hawthorn-sized) and medium (cherry-sized) artificial hosts, but strongly preferred medium hosts over large hosts. Size discrimination by R. indifferens females was independent of fly age.