The operation of a cyclone bioreactor differs from conventional stirred tanks since the agitation is accomplished by means of a pumped recirculation loop. Oxygen transfer can occur across the swirling gas-liquid interface in the cyclone or from bubbles entrained in the recirculation loop. A cyclone bioreactor was scaled-up from a 1 dm(3) bench top unit to a 75 dm(3) Process Development Unit (PDU). A reduction in the aspect ratio was compensated for by extending the length of the recirculation loop and providing additional aeration. Performance of the two reactors for the production of microbial poly-beta-hydroxybutyrate (PHB) was compared under various operating conditions. The culture used for PHB production was Alcaligenes eutrophus DSM 545, grown on a mineral salts medium limited by the supply of nitrogen. The levels of dissolved oxygen obtained in the PDU were strongly dependent on the location at which the air was introduced into the reactor. However, with aeration balanced between two injection points and a similar level of power input, 17 Js(-1) dm(-3), the PDU was able to provide at least as much oxygen transfer capability as the laboratory-scale reactor. Under all conditions tested, the PHB accumulation by A. eutrophus was in excess of 80% of the biomass dry weight, although the yield on glucose was lower in the PDU than in the laboratory-scale reactor.