The milk whey from a mature cheese factory deproteinised by acid thermic coagulation (pH 4.5 and 90 degrees C), provides a good culture media for the production of Kluyveromices fragilis biomass. The optimal experimental conditions for the maximal production of biomass were established by using fermenters with different capacity and design. For lactose concentration of 15 g/l, pH 4.5, 30 degrees C and aireation between 0.25 and 1 VVM, the duplication time was below two hours and 98 per cent of the lactose was consumed. The obtained yield in dried weight was between 36 and 49 per cent (g biomass/g lactose). The biomass (without broken cell) contain 46 per cent protein on dry base and showed an "in vitro" digestibility of 65 per cent. The organic mass decreased 80 per cent after 12 hour of fermentation. This process eliminates a polluting agent and simutaneously, produces a biomass that could have industrial use as a protein complement in feeds.