Valine dehydrogenase (VDH) is believed to be absent in Streptomyces avermitilis. In the present study, a VDH (M(r) 72 000) was detected by activity measurement and activity staining on a native-PAGE gel. The enzyme activity was induced by L-valine and repressed by ammonia. VDH activity was found to be significantly lower than L-valine transaminase activity. The results suggest that one active VDH does exist in S. avermitilis, and plays a role in valine catabolism and avermectin biosynthesis.