Abstract: Acetolactate synthase (ALS), a key enzyme in the biosynthesis of valine, leucine and isoleucine in plants is inhibited by herbicides comprising imidazolinones. A mutant gene encoding imidazolinone-resistant ALS has been isolated from an imidazolinone-resistant Arabidopsis thaliana (GH-90) and stably maintained in a plasmid pKS1. When compared with the gene coding for imidazolinone-sensitive ALS, the imidazolinone-resistant ALS gene contains a point mutation, wherein adenosine is substituted for guanosine at nucleotide 1958, resulting in the substitution of asparagine for serine at amino acid residue 653. The mutant ALS gene can be used to impart imidazolinone resistance to a crop plant; thereby permitting the utilization of the imidazolinone or analogous herbicide as a single application at a concentration which ensures the complete or substantially complete killing of weeds, while leaving the transgenic crop plant essentially undamaged.