The western cherry fruit fly, Rhagoletis indifferens Curran, is a frequent, chronic pest of sour cherry (Prunus cerasus L.) in western North America, whereas the apple maggot fly, R. pomonella (Walsh), uses this host rarely. We compared male territoriality and other aspects of host use by these species on sour cherry in the laboratory. When flies were released singly onto fruits, residence times of both sexes of R. indifferens were longer than those of R. pomonella, and females of R. indifferens attempted to oviposit much more frequently than did R. pomonella females. During conspecific encounters, males of R. indifferens interacted for longer periods of time than did R. pomonella males, were more active per unit time, and were more likely to engage in aggressive behavior (e.g., "pouncing"). One fly was actively driven off the fruit in 17% of trials involving R. indifferens males vs. 4% of trials involving R. pomonella males. In head-to-head encounters between species, R. indifferens males were more likely to be the sole fly remaining on a fruit. Fly size (as determined by maximum head width) did not appear to influence the outcome of conspecific encounters between R. indifferens males. Both phenological and behavioral differences between R. indifferens and R. pomonella appear to account for their different degrees of association with sour cherry in Utah.