The effects of prometryn [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], fluometuron [1,1-dimethyl-3-(a,a,a-trifluoro-m-tolyl)urea], and MSMA (monosodium methanearsonate) on Chlorella pyrenoidosa Chick ‘Strain 252′, Trichoderma viride Pers. ex Fr., and Aspergillus terreus Thom were investigated. Minimum herbicide concentrations necessary to decrease C. pyrenoidosa growth after exposure for 35 generations were 10-8M diuron, 10-7M prometryn, and 10-6M fluometuron. MSMA had no effect. C. pyrenoidosa photosynthesis was essentially stopped by a 2-hr exposure to 5 × 10-6M prometryn or diuron and by 5 × 10-5M fluometuron. Prometryn (5 × 10-5M) or diuron (5 × 10-7M) increased respiration by 70 and 44%, respectively after an 80-min treatment. Maximum fluometuron inhibition of respiration was ca 25% and MSMA had little or no effect. Herbicide concentrations of 0, 10-6, 10-5, and 10-4M were used to measure their effects on colony diameter growth, gain in mycelial dry weight, sporulation, and respiration of the two fungi. No concentration of any of the herbicides had any major effect on any of these parameters except sporulation. T. viride sporulation was increased approximately 20, 30, 35, and 50% by 10-6M concentrations of prometryn, diuron, fluometuron, and MSMA, respectively. Sporulation of A. terreus was also somewhat increased by these herbicides.