We investigated the mechanism for conferring herbicide resistance in transgenic rice. Plants from Line M4 were resistant to PROTOX inhibitors and had yields similar to those from wild-type (WT) rice.Myxococcus xanthus PROTOX mRNA was abundantly expressed in the transgenic leaf tissues, and theM. xanthus PROTOX gene was stably transmitted into the T4 generation. We detected a protein with a predicted molecular mass of 50 kD, equal to the weight ofM. xanthus PROTOX, in M4 but not WT plants. Furthermore, several PROTOX-inhibitor herbicides — acifluorfen, oxyfluorfen, carfentrazone-ethyl, and oxadiazon — caused significant cellular leakage and lipid peroxidation in the WT, but not in the transgenics. Total PROTOX activity in untreated transformed rice was 17-fold higher than in the WT, with activity being inhibited in the latter genotype by 55%, 59%, 53%, or 60% as a result of treatment with acifluorfen, oxyfluorfen, carfentrazone-ethyl, or oxadiazon, respectively. However, PROTOX activities in transgenic rice were similar to their corresponding, untreated controls. The accumulation of Proto IX was 15-to 21-fold higher in the WT than in M4 when plants were treated with PROTOX inhibitors. In the former, their epicuticular wax and chloroplasts were severely damaged after oxyfluorfen treatment The lack of damage in transformed plants suggests that M4 does not accumulate Proto IX, probably due to the production of herbicide-resistant chloroplastic and mitochondria PROTOX.