This study was aimed at examining the effect of hydraulic retention time (HRT) on the progress and efficiency of continuous biodegradation of sugar beet distillery stillage. Aerobic biodegradation of stillage was conducted in a 2-L stirred-tank reactor using a mixed culture of Bacillus bacteria under mesophilic conditions (36ºC). Continuous process was operated at HRTs of 39, 58 and 90 h. All experiments were performed at uncontrolled pH, stirrer speed of 900 rpm, and aeration of 1.0 vvm. The pH value of degraded stillage (5.25) was also not adjusted. The longer the HRT, the higher the process efficiency expressed by the sum of chemical oxygen demand (COD) determined after suspended solids separation and theoretical COD of betaine, as well as by five-day biological oxygen demand and total organic carbon. The greatest reduction in these terms was achieved at HRT of 90 h, i.e., 80%, 98% and 73%, respectively, at the COD removal rate of 0.3 g O₂/(L h). Only this variant of HRT ensured high assimilation of nitrogen (39%) and total phosphorus (69%) by bacteria. At the longest HRT, contents of reducing substances and organic acids decreased by 78% and 89%, respectively, while glycerol and betaine were completely removed.