The effects of imazethapyr, an imidazolinone derivative on the soil microbial biomass was investigated in a clay loam soil. Imazethapyr was applied in a field trial at the recommended field rate for soybean weeding, 50 g a.i. ha^-1. In laboratory experiments the herbicide was incorporated at the field rate and 10-fold and 100-fold higher. In both the field trials and the laboratory experiments, the field rate of imazethapyr had no adverse effects on the microbiological processes tested. But at 10-fold and 100-fold higher rates, the herbicide decreased the biomass-C content and dehydrogenase activity and increased the hydrolytic capacity and protease and catalase activity. These findings suggest that there may be a risk in monocultural practices that require repeated herbicide treatments.