The plasmid stability of a recombinant Saccharomyces cerevisiae strain, which expresses cloned alpha-amylase, was increased when glucose or yeast extract was fed with multi-pulse mode in fed-batch culture. Using a novel strategy combining constant rate fed-batch culture and multi-pulse feeding of yeast extract, the plasmid stability was maintained over 90%, meanwhile, 36 g cells l(-1) and 208 units of cloned alpha-amylase activity ml(-)1 were obtained.