The microbial transformation of a sesquiterpene lactone, l‐α‐santonin was carried out by Streptomyces roseochromogenes NRRL‐B‐1233. The product obtained from l‐α‐sanatonin was purified using silica gel column chromatography with 1% acetone‐benzene as the eluant. The molecular weight of the product was 248, estimated by mass spectroanalysis. The product was assignad the chemical structure of 1,2‐dihydro‐l‐α‐santonin on the bases of 1H and 13C‐NMR and mass spectro‐analyses. Similar products were also obtained from other actinomycetes. The conversion of l‐α‐santonin to 1,2‐dihydro‐l‐α‐santonin was 28% for Streptomyces roseochromogenes NRRL‐B‐1233, 14% for S. roseochromogenes ATCC 13400, 30.9% for S. aureofaciens KCC‐S‐0008, and 43.0% for S. aureofaciens KCC‐S‐0624, respectively.