T HE herbicides glyphosate, bromacil, and dalapon were assessed for their affect on the oxygen uptake of microorganisms from an aerated dairy lagoon at 20 °C. The maximum recommended addition of herbicide active ingerdient to a lagoon is: glyphosate (isopropyl amino salt)-0.42 mg/L; bromacil-3.70 mg/L; and dalapon (85.8 percent Na salt and 14.2 percent Mg salt)-1.95 mg/L. In addition, microorganism isolates from the aerobic dairy lagoon were incubated under aerobic and anerobic conditions at room temperature to assess the effects of the herbicides. Under aerobic incubation, it was found that neither herbicide type nor herbicide concentration affected isolate growth. Under anaerobic incubation, both inhibition and stimulation effects were found with gram-positive rod organisms being least affected.