Unlike laboratory-scale bioreactors, large reactors have finite dispersion and are affected by unexpected interruptions in the feed streams. For the optimum level of dispersion determined earlier, the effects of interruptions in the inflow of the carbon ( glucose) and nitrogen (NH4Cl) substrates have been studied for fed-batch fermentation by Ralstonia eutropha to produce poly-beta-hydroxybutyrate (PHB). Optimum uninterrupted operation by neural control at dispersion corresponding to Pe = 20 was used as the reference. While glucose stoppage mainly affected cell growth, nitrogen flow suppressed polymer synthesis more strongly. The effects depended also on the time at which the interruptions occurred. These results are interpreted in a metabolic context, emphasizing the complexity of the system and the importance of 'intelligent' monitoring and control under disturbed conditions.