Bench-scale batch bioreactors were used to study the effectiveness of cheese whey fermentation for single-cell protein production using the yeast Kluyveromyces fragilis in reducing the pollution potential of whey as measured by solids, chemical oxygen demand (COD) and nitrogenous compounds concentrations. The four principal phases (lag, exponential, stationary and death) encountered in the history of a microbial culture grown under batch conditions were clearly recognized in the growth, temperature and dissolved oxygen curves. The lactose concentration and soluble COD displayed three distinct phases corresponding to the lag, exponential and stationary phases of the yeast growth. The minimum dissolved oxygen and maximum temperature observed in this study (at an air flow of 3 VVM, a mixing speed of 400 rpm and an ambient temperature) were 2.49 mg/L and 31.6 degrees C, respectively. About 99% of lactose (90.6% of soluble COD) was utilized after 28 h. The total COD continued to decline due to cell death resulting in a reduction of 42.98%. The total nitrogen concentration remained unchanged while the organic nitrogen increased during the exponential phase and then declined during the death phase. The ash content remained unchanged while a substantial reduction (56%) of the volatile solids was observed. These results indicated that sufficient oxygen for yeast growth was present in the medium and no cooling system was needed for this type of fermenter under similar experimental conditions. Recovering the yeast biomass with ultrafiltration reduced the total COD by 98% of its initial value in the raw whey.