Aims: To examine the efficacy of natamycin produced by Streptomyces natalensis against strains of Aspergillus carbonarius growth and ochratoxin A (OTA) production under different environmental factors on a grape juice-based medium. Methods and Results: Detailed studies in the range 0-20 ng ml(-1) for control of growth and ochratoxin production by strains of A. carbonarius at 0.98, 0.96 and 0.94 water availabilities (a(w)) and 15-25 degrees C on a fresh red grape extract medium were examined. Inhibition of growth was depending on temperature and a(w) level. At 15 degrees C, 5-10 ng ml(-1) natamycin was effective in reducing growth almost completely. However, at 20-25 degrees C and all the three a(w) levels, growth was only slightly inhibited by 5-10 ng ml(-1) natamycin. There were strain differences with regard to inhibition of OTA production. At 15 degrees C and 0.98 a(w), 10 ng ml(-1) was required to inhibit production by > 90%. However, at 0.96 and 0.94 a(w), almost complete inhibition occurred. At 20 degrees C, OTA production was only significantly inhibited by 10 ng ml(-1) natamycin at 0.94 a(w). At 0.96 and 0.98 a(w), some inhibition occurred with 5-10 ng ml(-1), but greater concentrations would be required for effective inhibition. At 25 degrees C, 5 ng ml(-1) was effective at all a(w) levels. However, at 15 degrees C and 25 degrees C and a wide range of a(w) levels, natamycin effectively controlled OTA production. Conclusion: Natamycin appears to be a very effective for controlling growth and OTA production by strains of A. carbonarius over a range of a(w) and temperature conditions on grape-based media. Significance and the Impact of the Study: This is the first detailed study to demonstrate the impact of natamycin against A. carbonarius. This study suggests that use of natamycin at 50-100 ng ml(-1) can give complete inhibition of growth of A. carbonarius and OTA production over a range of environmental conditions. Natamycin could be an important component of a system to prevent OTA contamination of wine as well during the drying and production of vine fruits.