Abstract: This study investigated the toxicity of chemical dye production industry wastewaters and emphasized the importance of toxicity tests in wastewater discharge regulations. The dyestuff samples were obtained from an untreated effluent at Kafr El-Dawar (Egypt) during 5 months from December 2005 to April 2006. The effluent was near-neutral, strongly colored and highly ecotoxic with a COD content of 240 mg L-1. Catfish (Clarias lazera) were used as test organisms. The 96 h LC50 of C. lazera exposed to the effluent was found to be 35%. Twenty eight-day exposures to 3.12, 6.25 and 12.5% doses were conducted and several toxicological endpoints were evaluated. The predominant haematological change was a severe anaemia. Leucocytes count increased in stressed fish. Moderate neutrophilia and lymphopenia were detected in fish exposed to effluent concentrations of 6.25 and 12.5%. Monocytosis was quite evident at 12.5% concentration. Overall increases over control were observed in serum transaminases (GPT and GOT). However, these increases were not dose dependent. Blood urea insignificantly changed in treated fish while serum creatinine significantly elevated after exposure to 3.12 and 12.5% dose levels. In addition, free amino acids in flesh exhibited a decreasing trend at all concentrations tested. Moreover, histopathological alterations in the gills, liver and kidney occurred exclusively after treatment. Histological analysis of gill samples revealed a range of lesions including lamellar fusion due to hyperplasia and hypertophy of epithelial cells, subepithelial cell edema, collapsed pillar cell system and extensive lamellar aneurism. Liver pathologies included extensive necrosis of hepatocytes, cytoplasmic vacuolation, distended sinusoids with massive congestion and infiltration of inflammatory cells through out the liver parenchyma. In the kidney, the glomeruli appeared shrunken. The tubular epithelium was desquamated, vacuolated and often destroyed, which render the tubular system of the mesonephros incapable of functioning properly. From the present findings, it is assumed that under field conditions, the dyestuff effluent could induce general toxic effects and possibly might disturb ecologically relevant processes such as fish reproduction.