A specific regulatory effect of ferrous ion on a biosynthesis of mildiomycin by Streptoverticillium rimofaciens was investigated. The minimal concentration of ferrous ion necessary for the maximal production of mildiomycin was about 8 micrograms/ml. The physiological effects of ferrous ion on the mildiomycin production were examined in the media with and without the ferrous ion. Addition of ferrous ion to the culture medium increased both the mildiomycin production (10 times) and an assimilation of ammonium-nitrogen, activities of peptidase and protease.