Effect of the soil-applied herbicide atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) on growth activity of Sclerotium rolfsii and Trichoderma viride was determined in soil culture. The fungi were grown axenically in flasks of sterilized soil containing 0, 8, 20, 40, 80 μg of atrazine per gram of soil, and CO2 evolution was measured at intervals over a period of 19 days for the pathogen and 14 days for T. viride. For S. rolfsii, maximum CO2-carbon was produced in soil with 8 μg of atrazine, this being significantly above the O-herbicide control. Higher concentrations greatly inhibited activity of the pathogen. Correlation coefficients revealed that total production of CO2-C was closely related to amount of NO3-nitrogen consumed and titratable acidity produced (mostly oxalic acid); soil pH decreased with increased activity of the fungus. T. viride grew very rapidly in all treatments, activity being most accelerated between 3 and 10 days after inoculation. Production of CO2-C for all treatments above 8 μg indicated a stimulatory effect upon this fungus. Economic coefficients relating total CO2-C to amount of NO3-N consumed were highest for the 80-μg atrazine treatment and declined with decreasing concentration. Soil pH increased from an initial value of 6.4 to 7.4, with no significant differences between treatment.