Effects of mechanical and hydrodynamic forces on cells of the microalga Porphyridium cruentum are reported for aerated cultures in stirred vessels. The specific rate of cell death varied linearly with the specific interfacial area, suggesting the importance of bubble-cell contacting in the damaging process. At low rates of mechanical agitation, bubble break-up at the culture surface was the predominant cause of damage. In contrast, at higher agitation intensities, cell damage was caused predominantly by hydrodynamic stress in bulk turbulent flow. (C) 2000 Elsevier Science Ltd. All rights reserved.