Avermectinss,a group of antiparasitic macrolides produced by Streptomyces avermitilis.are widely used as anthelmintic agent in the agricultural,veterinary and medical fields [1] In the process of avermectin fermentation.starch is the most important carbon source.As starch cant penetrate the ceil mermbrane,many microorganisms secrete amylases that produce maltose and maltodextrins as major degradation products of starch,and the subsequent uptake of maltose or maltodextnn is usually mediated by maltose transport system.In the present study,the function of maltose transport system in S.avermitilis and its role in improving avermectin yield were investigated.