AbstractRaphanus raphanistrum L has evolved widespread resistance to sulfonylureas in the Western Australia (WA) wheat belt. With the introduction of imidazolinone‐tolerant (IT) wheat (Tritcum aestivum L) and IT canola (Brassica napus L) in the WA wheat belt, it is important to understand the status of cross‐resistance in this weed to sulfonylurea and imidazolinone (Imi) herbicides. A study was conducted to examine cross‐resistance between chlorsulfuron and Imi herbicides (a mixture of imazapic and imazapyr) in 46 R raphanistrum populations collected from across the WA wheat belt. Plants were treated with herbicides and assessed for phytotoxicity under glasshouse conditions. Of the 46 R raphanistrum populations, 32 were resistant to chlorsulfuron and four were resistant to imazapic + imazapyr. Of the 70% chlorsulfuron‐resistant populations, 13% showed cross‐resistance to imazapic + imazapyr. However, the cross‐resistant populations treated with imazapic + imazapyr showed a lower resistance level than the chlorsulfuron‐treated populations. These results suggest that weed populations with such cross‐resistance will not be controlled effectively by Imi herbicides. Although the resistance levels of the cross‐resistant populations to Imi herbicides were low, the cross‐resistance levels of R raphanistrum should be determined before growing IT crops, particularly IT canola.© 2002 Society of Chemical Industry