Residual herbicidal action of 3-amino-2,5-dichlorobenzoic acid (amiben), the methyl ester of amiben, 3-acetamido-2,5-dichlorobenzoic acid (N-acetyl amiben), 3-propionamido-2,5-dichlorobenzoic acid (N-propionyl amiben), 3-benzamido-2,5-dichlorobenzoic acid (N-benzoyl amiben), and the methyl ester of N-benzoyl amiben after 0, 1, 2, 4, and 6 hr irradiation was measured by oats (Avena sativa L., var. Markton) bioassay. In a greenhouse study, amiben and N-benzoyl amiben were applied to the soil surface, exposed to sunlight for 7.5 hr, and similarly bioassayed. Amiben, the methyl ester of amiben, and N-acetyl amiben lost 40 to 60% of their original herbicidal activity after 2 to 4 hr irradiation in the laboratory. N-benzoyl amiben and its methyl ester retained over 90% of their activity after 6 hr irradiation. Solar illumination reduced the herbicidal activity of amiben applied to the surface of wet soil by 11 to 14% but had no effect on N-benzoyl amiben. On a dry soil, the losses of amiben and N-benzoyl amiben were not significant.