The current work aims to study the biomass behaviour in a continuous mode activated sludge system (ASS) treating olive mill wastewater (OMWW) through an increasing OMWW food to microorganism ration (F/M). To this end, the biomass growth, the specific oxygen uptake rate (SOUR), microbial characterization, sludge volume index (SVI) as well as COD and phenolic compounds removal efficiencies were examined over time. Results showed a successful growth of the biomass that reached 6.79 g(TSS) l(-1) and 5.42 g(VSS) l(-1). Its viability, its adaptability, and its good physiological activity were confirmed by the obtained result of SOUR with an average of 9.95 mg(O2)gV(SS)(-1)h(-1), as well as aerobic microbial population characterization in terms of aerobic revivable bacteria at 22 degrees C and 37 degrees C, Pseudomonas sp., mould and yeast and total fungi. The concentration of these strains characterized by their ability to degrade effectively COD and phenolic compounds increased significantly (p < .05) over time. This demonstrated a great promptness in response to the increasing OMWW mass ratio. For all treatment steps, removal efficiencies were high and reached 95% of COD and 93% of phenolic compounds, also the flocs settleability shown by SVI measurement was optimal.