Nanaomycin production by Streptanyces rosa subsp. notoensis in complex media was inhibited by exogenously supplied inorganic phosphate. The inhibition was reversed by phosphate-trapping agents such as allophane and aluminum oxide. Under such condition nanaomycin production increased to the control level, and the phosphate content dropped down to the unsupplemented level. When allophane was added to conventional complex media containing nutrient-derived inorganic phosphate, the production of nanaomycin and several other antibiotics, which are subject to phosphate regulation, was enhanced several fold with the simultaneous reduction of free phosphate. The term "phosphate-depressed fermentation" is proposed for this technique.