A detailed analysis of the motion af a single particle in a slowly rotating horizontal cylinder in the avalanching mode has been carried out using the positron emission particle-tracking technique to follow the motion of a tagged particle. Cylinders of diameters 240, 390, and 488 mm, and sand of mean diameter 500 mum were used; the tracer was an irradiated sand particle. It was found that a tracer particle rotates with the bed in rigid body motion and then traverses the active surface region in a series of discrete avalanches before being absorbed once again into the rigid body rotation. The mean number of avalanches required to traverse the active region decreased from 3.5 to 4 at low rotational speed to I at higher rotational speed at the transition to the rolling mode. (C) 2003 Elsevier B.V. All rights reserved.