Plants, plant parts and plant seeds that are resistant to imidazolinone herbicides are provided. Plants are disclosed that contain a mutation in an AHAS gene. Specifically, plants are disclosed that contain a mutant AHAS gene allele of the Brassica juncea B genome. Two B. juncea AHAS gene sequences (BjAHAS-a and BjAHAS-b) and one B. nigra AHAS gene sequence (BngrAHAS) are disclosed. The sequence of the mutant allele, BjAHAS-bR, is also disclosed. Various methods are disclosed that include creation of mutant B. juncea lines, selection for herbicide resistant lines and determining the presence of the BjAHAS-bR mutant allele after crosses.