The neural pathways by which information about the acoustic world reaches the auditory cortex are well characterized, but how auditory representations are transformed into motor commands is not known. Here we use a perceptual decision-making task in rats to study this transformation. We demonstrate the role of corticostriatal projection neurons in auditory decisions by manipulating the activity of these neurons in rats performing an auditory frequency-discriminationtask. Targeted channelrhodopsin-2(ChR2)(1,2)-mediated stimulation of corticostriatal neurons during the task biased decisions in the direction predicted by the frequency tuning of the stimulated neurons, whereas archaerhodopsin-3 (Arch)(3)-mediated inactivation biased decisions in the opposite direction. Striatal projections are widespread in cortex and may provide a general mechanism for the control of motor decisions by sensory cortex.