Abstract: Pichia stipitis yeast cells were immobilized by entrapment in marine alga aluminum gel,the resulted gel beads with a diameter of 2～3 mm were further incubated.Yeast cells were densely immobilized on the surface of the gel beads,therefore decreased the resistance to mass transfer and benefited the semiaerobic fermentation.The proper initial pH value of sugar solution for ethanol fermentation with immobilized cells was 5.0～5.5.In the fermentation of 60 g/L glucose xylose mixture,glucose was mainly consumed in the first 12 h,followed by xylose utilization in the next 12 h.In order to reduce bacterium contamination after several cycles of fermentation,the yeast cell immobilized gel beads were treated at pH 2.8 for 2～3 h,followed by normal pH fermentation.Most bacteria were killed after such a treatment,while the fermentability of the immobilized cells was not affected significantly.When treated at pH 2.4 or bellow,however,the immobilized cells were injured and it was difficult to recover the fermentability.