Among various micro-organisms screened for the stereoselective reduction of 4-chloro-1-(4-fluorophenyl)butan-1-one (1), Hansenula polymorpha [American Type Culture Collection (A.T.C.C.) 26012 and 86014], Nocardia salmonicolor [Squibb Culture (S.C.) 6370], Arthobacter simplex (A.T.C.C. 6949), Mycobacterium vaccae (A.T.C.C. 29678), Candida boidinii (A.T.C.C. 13821) and Saccharomyces cerevisiae (A.T.C.C. 13792) reduced compound 1 to the corresponding (R)-(+)-alcohol (2). In contrast, Lactobacillus kefir (A.T.C.C. 35411), Pullularia pullulans (A.T.C.C. 16623), Trigonopsis variabilis (A.T.C.C. 10679) and Cunninghamella echinulata (A.T.C.C. 26269) reduced compound 1 to the (S)-(-)-alcohol (2). When 1-(4-fluorophenyl)-4-(1-piperazinyl)butan-1-one (3) was used as substrate for the reduction, only Nocardia globerula (A.T.C.C. 12505) and Saccharomyces cerevisiae (A.T.C.C. 13792) converted compound 3 into the corresponding (R)-(+)-alcohol (4). Organisms which reduced compound 1 were inactive for the reduction of compound 3. 1-(4-Fluorophenyl)-4-[4-(5-fluoro-2- pyrimidinyl)butan-1-one (5) was reduced to the corresponding (R)-(+)-alcohol (6) by Mortierella ramanniana (A.T.C.C. 38191) and to the (S)-(-)-alcohol (6) by Pullularia pullulans (A.T.C.C. 16623). (R)-(+)-compound 2 and compound 4 are key chiral intermediates in the total chemical synthesis of (R)-(+)-compound 6, an effective antipsychotic agent under development at Bristol-Myers Squibb. A single-stage (fermentation/biotransformation) process and two-stage (fermentation and subsequent biotransformation by cell suspensions) process were developed for the stereoselective reduction of compound 5 to (R)-(+)-compound 6 by Mortierella ramanniana (A.T.C.C. 38191). In both processes, the reaction yield of 98% and the optical purity of 99.4% were obtained for (R)-(+)-compound 6. The enzyme which catalysed the reduction of compound 5 to (R)-(+)-compound 6 was purified to homogeneity. The purified protein consisted of a single polypeptide of 29 kDa.