Territory defense is considered one of the primary functions of bird song, but this hypothesis has been directly tested in only a few cases. We used the speaker replacement method to ask whether song functions as a 'keep out' signal in song sparrows, a species for which there is considerable evidence supporting a mate attraction and stimulat on function of song, but only indirect evidence that song functions as a signal to other ma es. We removed 11 matched pairs of male song sparrows from their territories, replacing one male of each matched pair with loudspeakers broadcasting that male's song (the 'experimental' territory) while leaving the other male's territory silent (the 'control' territory). In all cases in which encroachments or takeovers occurred, these occurred first (or solely) on the control territory of a matched pair, supporting the hypothesis that song functions in territory defense in this species. The incidence of intrusions on control territories was very low, however, posing difficulties for the interpretation of speaker replacement experiments designed to ask more specific questions about how song functions in male-male aggressive competition.