Triazine herbicides inhibit photosynthesis by blocking electron transport in photosystem II. The target site of the herbicide was identified as a chloroplast thylakoid polypeptide (the Qв protein) of 32,000 daltons. Studies of triazine-resistant weed biotypes suggested that a subtle change in the Qв protein caused the resistance. We have cloned the chloroplast gene (psbA) that codes this protein from herbicide-resistant and herbicide-susceptible biotypes of Solanum nigrum. By DNA sequencing we detected a single base substitution in the psbA gene of the resistant plants, resulting in an amino acid change (serine to glycine for the susceptible to resistance conversion). This mutation is exactly the same one which we have described in a herbicide-resistant biotype of Amaranthus hybridus.