Positron emission particle tracking has been used to track the motion of a single radioactively labelled tracer particle within a bed of similar particles in a partially filled horizontal rotating drum. Runs were performed using 1.5 mm glass spheres in a 136 mm diameter drum and using 3 mm glass spheres in 100 and 144 mm diameter drums, at drum rotation speeds from 10 to 65 rpm. An active surface layer approximately two-thirds as thick as the underlying bed layer was apparent in all cases. Considerable slip of the bed at the walls was observed in most runs, which is attributed to a rolling motion of the outermost layer of spheres. The axial dispersion coefficient was determined for each run and was found to be proportional to the frequency at which particles circulate around the bed, and to be strongly dependent on particle size but independent of drum diameter.