The effect of chlorotoluron on different slender foxtail populations that survived normal agricultural rates of this herbicide (2.5 to 3.5 kg ai ha–1) was investigated under controlled laboratory conditions. Among five populations tested, two had tolerance for the herbicide. The ED50 values for these biotypes ranged from 0.33 to 2.43 kg ha–1. Assays with radiolabeled 14C-chlorotoluron indicated similar absorption and translocation of the herbicide between susceptible and resistant biotypes. Chlorophyll fluorescence and Hill reaction analysis seem to support the view that the mechanism of resistance of one biotype to the herbicide is due to degradation/detoxification rather than to modification of the target site, although compartmentation processes cannot be ruled out. This resistant biotype had high tolerance to other photosynthetic inhibiting phenylureas, including linuron, chlorbromuron, diuron, monolinuron, and neburon.