The mixing and motion of particles were analyzed in a rotary drum as part of the development of a solid-state fermentation bioreactor for spore production from the fungus Metarhizium anisopliae. Rice was chosen as the particle because it is often used as a substrate for fungal incubation. The controlled variables were the particle load, drum spin velocity and the presence of inner tubes. The dyed-tracer technique was applied to the system, and image analysis was performed using the commercial software LensEye. A detailed calibration procedure is described. The software was able to identify particle weight concentrations of up to 20% with good accuracy. The load of particles was the only variable to influence the number of revolutions necessary to obtain a uniform mixture: 5,9 and 11 rotations were required to homogenize 5, 10 and 15 kg of particles, respectively. The presence of inner tubes strongly influenced the particle velocities in the central core of the bed and changed the velocity patterns in all regions. (C) 2012 Elsevier B.V. All rights reserved.