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In situ product crystallization was investigated for solid product crystals that were obtained during fermentation. The model reaction was the asymmetric reduction of 4‐oxoisophorone (OIP) using baker's yeast (S. cerevisiae) as a biocatalyst. The target product was 6R‐dihydro‐oxoisophorone (DOIP), also known as levodione, a key intermediate in carotenoid synthesis. DOIP was degraded by baker's yeast mainly to (4S,6R)‐actinol, an unwanted byproduct in the process. Actinol formation reached up to 12.5% of the initial amount of OIP in the reactor during a batch process. However, better results were obtained when the dissolved DOIP concentration was controlled using an integrated fermentation–crystallization process because: (a) actinol formation was reduced to 4%; and (b) DOIP crystal formation in the reactor was avoided. DOIP productivity was improved by 50% and its selectivity was raised from 87% to 96 …