The actinomycete DEM20745, collected from non-rhizosphere soil adjacent to Paraserianthes falactaria trees (Cangkringan, Indonesia), is an efficient producer of the anticancer ansamycin polyketide 17- O -demethyl-geldanamycin (17- O -DMG), a biosynthetic precursor of the Hsp90 inhibitor geldanamycin (GDM). In DEM20745, 17- O -DMG is the major ansamycin product observed reaching a maximum titre of 17 mg/L in the fermentation broth. 17- O -DMG has the potential to be a key starting material for the semi-synthesis of GDM analogues for use in anticancer therapy. Thus, this preferential biosynthesis of 17- O -DMG facilitates easy access to this important molecule and provides further insight in the biosynthesis of the geldanamycins.