The aim of this study was to investigate the effect of pre-ozonation on soluble inert COD (SI) and soluble residual metabolic products (Sp) generated in the biological treatment of a textile industry wastewater. The characteristics of the wastewater samples (S1 and S2) taken randomly from the process wastewater balancing tank were as the following: Total COD (C-TO) = 1685-1700 mg/l, soluble COD (S-TO) = 950-1000 mg/l, BOD5=140-150; total suspended solids (TSS) = 250-750, volatile suspended solids (VSS) = 190-330, Total Kjeldahl Nitrogen (TKN) = 16-70 mg/l, and color = 1000-3300 Pt-Co units. A previously developed experimental method was used for the direct assessment of the inert COD fractions of raw and ozonated wastewater samples. 9.6 and 11.2 mg/min of ozone dose were applied during 30 min oxidation for S1 and S2 respectively. The toxicity of raw and pre-ozonated inert COD determination reactor effluents was tested using new born (< 24 h) Daphnia magna for 24 h exposure time.