Process for the cultivation of yeast cells

In a continuous process for the cultivation of yeast cells in an aqueous growth medium and under aerobic conditions, in which an aqueous nutrient medium comprising a source of assimilable nitrogen and methanol as a source of assimilable carbon is continuously delivered to a fermenting vessel containing the growth medium, and in which a stream of growth medium is continuously discharged from said vessel with recovery of the yeast cells present therein, the improvement which comprises using a yeast of the species Candida boidinii and using a nutrient medium comprising a primary source of assimilable nitrogen consisting of urea and a secondary source of assimilable nitrogen consisting of one or more compounds chosen from the group consisting of ammonia, ammonium hydroxide and ammonium salts, the ratio between nitrogen in the primary source and that in the secondary source being from 2:1 to 1:20.