Abstract: The invention relates to the isolation and modification of nucleic acid sequences encoding p-hydroxyphenylpyruvate dioxygenase enzyme from plants. These nucleic acid sequences were used to establish methods of identification of new herbicidal compounds that inhibit the activity of this enzyme, and to prepare new crop plants that are tolerant to the herbicidal action of inhibitors this enzyme. Chimeric genes comprising nucleic acid fragments containing all or part of the nucleic acid sequences encoding p-hydroxyphenylpyruvate dioxygenase may be used to produce active plant p-hydroxyphenylpyruvate dioxygenase enzyme in microorganisms, and to cause the production of modified forms of the enzyme in plants that may render such plants tolerant to inhibitors of the enzyme.