The toxicity of four xenobiotic compounds 3,5-dichlorophenol, formaldehyde, 4-nitrophenol and dichloromethane, representative of industrial wastewater contaminants was evaluated by a simple respirometric procedure set up on the basis of OECD Method 209 and by the Microtox bioassay. Very good reproducibility was observed for both methods, the variation coefficients being in the range of 2-10% for the respirometric procedure and 6-15% for Microtox, values that can be considered very good for a biological method. Comparison of EC(50) data obtained with the two methods shows that in both cases 3,5-dichlorophenol is more toxic than other compounds investigated and dichloromethane has a very low toxicity value. Intermediate EC(50) values were found for the two other chemicals, formaldehyde and 4-nitrophenol. Moreover, the Microtox EC(50) values are generally lower (except for dichloromethane) than the respirometric ones: these differences could be explained by the fact that the Microtox method uses a pure culture of marine species and, therefore, should not necessarily be expected to behave like a community of activated sludge bacteria. In conclusion, both methods can be usefully applied for toxicity detection in wastewater treatment plants but it is advisable to take into account that Microtox is more sensitive than respirometry in estimating the acute toxicity effect on the biomass operating in the plant.