AbstractThin‐layer and gas‐liquid chromatography, ultraviolet analysis and bioassay withChlorellaspp. have been used to investigate the pathway of degradation of diuron to phytotoxic derivatives when diuron was used as a soil‐residual herbicide in irrigation canals. Observations suggest that 1‐(3,4‐dichlorophenyl)‐3‐methylurea and 1‐(3,4‐dichlorophenyl)urea make a contribution to total residues equivalent to a maximum of about 40 and 55%, respectively, of diuron concentrations. Application of a phyto‐toxicity rating suggests that in this environment, measurement of diuron specifically would underestimate the total phytotoxicity of residues by a