Streptomyces avermitilis produces the anthelmintic and insecticidal secondary metabolite avermectins. Following mutagenesis of a recombinant strain, S. avermitilis K2038 (aveD X) with N-methyl-N'-nitro-N-nitrosoguanidine, a derivative strain K2057 (aveD aveE X), was isolated, which produced seven avermectin-related compounds different from the eight components of avermectins. Four among these seven compounds from mutant K2057 were found to be new metabolites. Their structures were 4'-deoleandrosyl-6,8a-seco-6,8a-deoxyavermectin B1a, 4'-deoleandrosyl-6,8a-seco-6,8a-deoxy-5-oxoavermectin B1a, 4'-deoleandrosyl-6,8a-seco-6,8a-deoxy-5-oxoavermectin B2a and 6,8a-seco-6,8a-deoxy-2,5-didehydroavermectin B2a, all of which lack the furan ring at C-6, C-8a. The mutation affecting the formation of the furan ring (aveE) is located in the center of the gene cluster for avermectin biosynthesis.