Parkinson's disease (PD) patients and their normal controls performed two experiments involving a sequential movement task, depressing a series of buttons at choice points along a response board. Visual or auditory cues were presented prior to each move according to various contingencies. PD, a disorder characterised by degeneration of the basal ganglia (BG), typically manifests with poor execution of motor sequences. We found that external cueing facilitated motor sequencing in PD patients. In particular, auditory cues which occurred late in the movement cycle maximally facilitated switching between subcomponents of a sequence. Based on physiological findings reported in the primate literature [Brotchie et al., Brain 114, 1685-1702, 1978; Schultz and Romo, Exp. Brain Res. 1, 363-384, 1992], it is suggested that external cues enhance performance by replacing defective, internally generated cues (discharges) of the BG. This has implications for the use of physical training strategies in the treatment of PD.