This study deals with the importance of chemical oxygen demand (COD) of the inert fractions of a textile finishing industry wastewater and the relation between them and the effluent's acute toxicity. The characteristics of three wastewater samples taken randomly were as follows: Total COD (C-TO) = 1250-1600 mg/l, total soluble COD (S-TO) = 800-950 mg/l, total suspended solids (TSS) = 250-750 mg/l, volatile suspended solids (VSS) = 175-330 mg/l, Total Kjeldahl nitrogen (TKN) = 16-70 mg/l, colour = 10003300 Pt-Co units. The inert soluble fractions of the samples were found in the same order as 15-16% in total COD, although the raw materials used in the process changed. The sum of the inert soluble COD (SI) and microbial product (S-P) was 16-17% of the total COD. These results were in the range of the previous studies indicating the biological treatability of the textile finishing industry wastewater, although it would be difficult to comply with the 200-250 mg/l of COD limit. The acute toxicity test conducted on raw wastewater samples and on the reactor effluents of both total and soluble inert COD fractions, filtered through 0.45 mum Milipore filter, showed that the toxicity was related to the inert soluble COD.