The performance of a plot scale upflow anaerobic sludge blanket reactor treating simulated cotton textile wastewater was examined to understand the effect of salt concentration in the color and COD removal efficiencies. The salt (NaCl) concentration increased from 0 to 128 g l(-1) while HRT. and the organic loadings were kept constant giving a HRT of 20 h and a 3.86 of volumetric organic loading rates. COD removal efficiencies decreased up to 18 from 80% as methanogenic bacteria inhibited by increasing salt concentration in the feed. Hi-h volatile fatty acid concentrations, low pH and high volatile fatty acids/bicarbonate alkalinity ratios in the effluent of reactor show the inhibition of methanogenic bacteria. A salt concentration of 128 g l(-1) cause to reduce methane percentage and methane production rates to 30% and 180 ml/day from 70% and 2260 ml/day. Although COD removal efficiencies decreased, However, the color removals was not effected (decolorization efficiency = 100%) with increasing NaCl concentration as high as 128 g l(-1) in the feed. (C) 2004 Elsevier Inc. All rights reserved.