The purpose of the present work was to study the influence of a non-ionic surfactant, a nonylphenol with four ethoxylated units (NP4EO), on the activity of the microbial population present in a laboratory-scale activated sludge unit. Traditional control methods for this type of unit were used (measurement of suspended solids and chemical oxygen demand) as well as specific techniques for the measurement of bacterial activity (dehydrogenase activity and specific oxygen uptake rate) and the results were compared. It was shown that the Specific Oxygen Uptake Rate (SOUR) is the simplest and quickest way to carry out routine control of activated sludge activity, while measuring dehydrogenase activity provides more complete control. The results obtained indicated that there was no inhibition of microbial activity at a concentration of 5 ppm of NP4EO, which was not the case with a concentration of 10 ppm.