Triazine resistance in a mutant biotype of Phalaris paradoxa is accompanied by changes in the chlorophyll fluorescence induction curve, and by reduced quantum yield for electron transport, indicating altered photosystem II activity. However, light-saturated rates of electron transport in isolated chloroplasts, rates of CO2 uptake in leaves and dry weight production of the triazine resistant biotype, are equal or superior to those of the wild type. A single mutation in the psbA gene, leading to a serine to glycine shift at position 264 of the thyiakoid membrane 32 kDa Qв- protein. was found in the herbicide resistant mutant. The results indicate that triazine resistance is not necessarily linked to inferior photosynthetic and growth performance.