||Two laboratory-scale hybrid biological reactors (HBRs) containing both suspended and attached biomass were used for treating a synthetic dairy waste. The relative specific biological activities between the two phases of biomass were evaluated. The suspended growth was maintained at a concentration up to 2500 mg/L (MLSS) by recycling the settled sludge while the attached growth (biofilm) was maintained at between 100 and 600 mg/L by using shredded automotive tyre beads (1 to 3 mm in diameter) as biocarriers, which were added at a concentration of up to 15 g/L in the biological reactor. The synthetic dairy waste was fed continuously at an increasing concentration from 300 mg/L to 1100 mg/L as COD, which corresponded to a volumetric organic loading of 0.9 kg COD/m3-day to 3.3 kg COD/m3-day. Throughout the study, the COD removal was over 80%, of which a significant fraction was contributed by the attached growth. In general, the specific bioactivity (expressed as mg O2 uptake per g sludge-hour) of the attached biomass was somewhat higher than that of the suspended growth at a given substrate concentration. Thus, the actual percent of waste stabilization contributed by the attached growth was somewhat higher than its biomass percentage in the reactor. Keywords: hybrid biological reactor, attached-growth, suspended growth, biocarrier, specific bioactivity, waste treatment.