A cluster encoding genes for the biosynthesis of meilingmycin, a macrolide antibiotic structurally similar to avermectin and milbemycin alpha11, was identified among seven uncharacterized polyketide synthase gene clusters isolated from Streptomyces nanchangensis NS3226 by hybridization with PCR products using primers derived from the sequences of aveE, aveF and a thioesterase domain of the avermectin biosynthetic gene cluster. Introduction of a 24.1-kb deletion by targeted gene replacement resulted in a loss of meilingmycin production, confirming that the gene cluster encodes biosynthesis of this important anthelminthic antibiotic compound. A sequenced 8.6-kb fragment had aveC and aveE homologues ( meiC and meiE) linked together, as in the avermectin gene cluster, but the arrangement of aveF ( meiF) and the thioesterase homologues differed. The results should pave the way to producing novel insecticidal compounds by generating hybrids between the two pathways.