Production of a new methylated 6,8a-seco-6,8a-deoxy derivative of the avermectins by a transformant strain of Streptomyces avermitilis.

6,8a-deoxy derivatives of avermectins produced by a mutant strain of Streptomyces avermitilis K20571} derived by mutation from strain K2038 producing avermectins B l a and B2a selectively2). The parent strain K2038 carries two types of mutations, C-5 0-methylation (aveD) and the selective incorporation of branched-chain fatty acid into the avermectin molecule (X). The mutant strain K2057 has an additional mutation (aveE) affecting furan ring formation at C-6 and C-8a positions besides both mutations aveDand X. Wecould not detect methylated 6,8a-seco-6,8a-deoxy derivatives of avermectins from mutant K2057 because the mutant lacks functions