The present invention provides methods to confer resistance to protoporphyrinogen-inhibiting herbicides onto crop plants. Resistance is conferred by genetically engineering the plants to express cloned DNA encoding a protoporphyrinogen oxidase resistant to porphyric herbicides. If such resistant crop plants are cultivated, utilization of these herbicides on fields of these crop plants becomes feasible. This should allow for simpler and more effective weed management, and increase the value of these herbicides for agricultural use. Furthermore, the present invention provides plants, algae, plant cells, and algal cells which have been made resistant to protoporphyrinogen oxidase-inhibiting herbicides by the subject methods using a herbicide-resistant protoporphyrinogen oxidase gene that has been prepared by genetic engineering methods. In addition, the present invention provides methods to evaluate the inhibitory effects of test compounds on protoporphyrinogen oxidase activity, as well as methods to identify protoporphyrinogen oxidase inhibitors among test compounds. Preferred cloned DNA fragments encoding protoporphyrinogen oxidase enzymes resistant to porphyric herbicides are also described.