Twenty-one wastewater samples were collected from 19 petrochemical plants and two united wastewater treatment plants located in southern Taiwan during 1991. Water samples were passed through an XAD-2 resin column and the resin was then eluted by acetone. The mutagenicity of the wastewater was determined by the Ames Salmonella/microsome test. A total of 52.4%of the wastewater samples in the survey were found to exhibit mutagenic activity in Salmonella typhimurium TA98 and TA 100 in the presence and absence of S9 mixture. Three samples among the 21 samples, had very high levels of mutagenicity. The measurement and identification of mutagenic pollutants in wastewater need further investigation.