Abstract: The invention relates to the microbial synthesis of polyhydroxyalkanoic acids [PHA(B)] from substrates which have potential environmental toxicity and must therefore be detoxified and which, if capable of being utilized as a source of carbon and energy by microorganisms for growth and propagation, exhibit the phenomenon of substrate inhibition. According to the invention, a process for the continuous microbiological production of PHB is provided, which process is characterized in that strains of microorganisms known to be PHB producers are propagated chemo-statically at maximum heat production on substrates exhibiting growth inhibition in case of excess substrate, the maximum heat production corresponding to a maximum PHB content in the biomass being adjusted by means of the substrate flow rate.