The invention relates to a process for producing a microbial storage compound, in particular polyhydroxyalkanoate, using micro-organisms capable of accumulating such microbial storage compound, wherein such micro-organisms are selected and the microbial storage compound is accumulated by carrying out the so-called feast phase of the selection step and the accumulation of the microbial storage compound in selected micro-organisms in the same reactor and carrying out the so-called famine phase of the selection step in a separate, smaller, reactor.