In this work, several batch fermentations by free and immobilized genetically modified S.cerevisiae pRN5 with amplified exo-β-glucanase (EXG) activity and a defective EXG1 gene strain S.cerevisiae MAX18-9B were carried out. The aim was to study the plasmid stability and the enzymatic expression in selective and non selective media. Batch experiments were developed in 250 ml Erlenmeyer flasks containing 100 ml of medium at 30°C, buffered at pH 5 and agitated at 150 rpm in an orbital shaker. Cells were immobilized by entrapment in Ca-alginate. When immobilized cells were grown in a non-selective medium, almost an 80% of cells maintained the plasmid after a batch process of 20 h while up to a 95% was achieved in a selective medium. However in free cell cultures, only a smaller fraction were recombinant cells (45%) after a quite short period of 17 h. Free cultures presented a maximum specific growth rate ( max ) 11.5% higher to that of immobilized cells, which can be correlated with a decrease in the fraction of plasmid bearing cells. The highest max was obtained for S. cerevisiae Exglacking in exo-β-1,3-glucanase activity. © 1998 Elsevier B.V. All rights reserved.