Summary.Immediate and residual phytotoxicity of binary combinations of several residual soil‐applied herbicides at 0·1 and 0·5 ppm was compared to the expected response calculated by Colby's formula. In one experiment diuron was combined with diphenamid, fluometuron, noruron, prometryne or simazine, and in another experiment trifluralin was combined with diuron or dichlobenil. The observed phytotoxicity of most combinations was only slightly different from the expected response. One combination, diuron 0·5 ppm + fluometuron 0·5 ppm, affected mustard significantly more than expected and this synergistic interaction remained appreciable after incubation of 2 months.Interactions entre les herbicides persistants a faible concent