A mutant strain of Yarrowia lipolytica was developed which produced 8.0 g L-beta-hydroxybutyric acid l(-1) from butyric acid in a batch culture. The optimum culture conditions in the fermenter for maintenance of a high cell activity, determined by chemostat analyses, were a specific growth rate of 0.06 h(-1), a glucose concentration of 2.0 g l(-1), and a butyric acid concentration of 8.1 g l(-1). A fed-batch fermentation was performed under these conditions resulting in an L-beta-hydroxybutyric acid yield of 31 g l(-1).