It has been suggested that interference in symbolically cued bimanual reaction time tasks is caused primarily by the perceptual processing of stimuli, and not by motor preparation of the required movements. Here, subjects made movements of the right and left index fingers that varied in their spatial and motor congruence. Spatial congruence was manipulated by presenting symbolic cues (i.e., pairs of letters) on a computer screen cueing the required movement directions. Motor congruence was manipulated by altering hand orientation. Results showed that interference occurs at both the stage of stimulus processing and the stage of motor preparation. These effects were reflected in the latencies of the different bimanual movements, with both motor incongruence and spatial incongruence causing significant increases in reaction time. However, spatially incongruent movements that were made in response to incongruent visual cues demonstrated changes in reaction time that were more than double those of movements that required simultaneous activation of homologous muscles. Therefore, in symbolically cued bimanual reaction time tasks, although both motor and spatial constraints operate, there is a clear dominance of spatial incongruence on performance. Whilst motor incongruence effects are likely due to cross-facilitation in corticospinal pathways, spatial incongruence effects are probably due to interference between the mechanisms that identify incongruent stimuli and translate these cues into the appropriate movements.