The potential extensive use of certain new herbicides has made it desirable to study their effects on soil microorganisms. If these compounds have any retarding or stimulating influences on the general soil microflora, this could be reflected in carbon dioxide evolution during soil respiration experiments. Microorganisms are important to soil fertility because they transform potential plant nutrients into active or available forms, and carbon dioxide is a significant index of their activity. The effects on carbon dioxide evolution of four new herbicides that have shown promising results in field trials at this station are presented in this report.