Lolium rigidum is a cross-pollinating grass weed present in Europe and occurring in winter wheat and orchard crops. Several graminicides such as chlorotoluron and/or isoproturon and diclofop-methyl in mixtures or alone have been used successfully to control this weed in Spain during the past decade. However, several L rigidum populations have developed resistance to these herbicides following selection due to their continuous use. Four resistance mechanisms have been found in this grass weed, an enhanced metabolic detoxiflcation of the herbicides and an insensitive isoform of ACCase being the most important ones. The extent of cross-resistance depends on the type of mechanism. The biotype with an enhanced metabolic detoxification showed cross-resistance to ACCase-, ALS-, PSII- and tubuline-inhibiting herbicides, while the biotype resistant due to a mutation of the target site (ACCase) presented cross-resistance to ACCase-inhibiting herbicides only.