Streptomyces sp. P6621 was very sensitive to linoleic acid when soybean oil was used as the sole carbon source for the production of cephamycin C. Cell growth was strictly restrained by adding 0.25 g/l of linoleic acid to the culture. An ultraviolet-induced mutation was performed based on the hypothesis that a mutant resistant to linoleic acid could produce antibiotics more efficiently. Mutant strain U-12-2 resistant to 0.4 g/l of linoleic acid was obtained by screening in solid and liquid media containing linoleic acid. The uptake rate of linoleic acid by U-12-2 was three times higher than that by the parental strain. Cephamycin C production from 30 and 60 g/l of soybean oil as the sole carbon source was 1.6 fold higher with the mutant strain compared to the parental strain. It is concluded that this mutant which is resistant to linoleic acid has improved cephamycin C production.