We have isolated several herbicide-resistant cell lines from photosynthetic cell suspensions of soybean (Glycine max) that possessed different levels of herbicide resistance, photosystem II activity, and chlorophyll a/b ratio. We have further studied the STR7 mutant, which showed the highest level of resistance to atrazine as well as a cross-resistance to 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (50- and 3-fold, respectively, compared with the wild type). Sequencing of the psbA gene (coding for the D1 polypeptide of photosystem II) from this mutant revealed a single change, serine-268 to proline, in the D1 protein. To our knowledge, this substitution has not previously been described in any photosynthetic organism. In addition to affecting atrazine resistance, this single amino acid change caused a decrease in the electron transfer rate between the secondary acceptors QA and QB and a stabilization of the S2QB- and S3QB- states. The mutant also showed a larger antenna size, an increase in non-QB-reducing centers, and a higher sensitivity to light stress. The unusual stability of the S2QB- and S3QB- states indicates that STR7 belongs to a new class of QB-site mutants.