The influence of the lactose content in the medium and of the air flow rate on the biomass yield of the yeast Kluyveromyces marxianus on whey substrate was investigated. For this purpose, batch processes were conducted in a laboratory bioreactor (2L) with the yeast Kluyveromyces marxianus ZIM75 in media with different contents of lactose and at different air flow rates. The higest biomass yield (5,9 g d.m./L) was achieved at the highest air flow rate (1,5 L/L min) in the medium with 10% of lactose. The specific growth rate increased with increased air flow rate, and the highest specific growth rate of 0,1060 h-1 was achieved at the highest air flow rate in the medium with 5% of lactose. However, the best conversion of lactose carbon to biomass (0,104 g/g) was achieved in the medium with 5% of lactose.