<jats:p> A study was made on the mineral requirements of <jats:italic>Streptomyces fradiae</jats:italic> strain 3535 for neomycin production. It was observed that optimal levels of the elements Ca, Fe, and Zn per milliliter of a synthetic medium for neomycin production were 10.8, 1.0, and 0.115 μg, respectively. K <jats:sub>2</jats:sub> HPO <jats:sub>4</jats:sub> was required at a concentration of 0.1% for maximal yield of neomycin, whereas NaCl and the metals Mn and Cu were without any effect. High doses of Zn (0.23 μg/ml or above) caused destruction of neomycin after the fifth day of fermentation.