A decaploid tall fescue (Festuca arundinacea Schreb) genotype has been found which exhibits net photosynthetic rates of 32 to 41 mg CO(2)/dm(2).hour as opposed to a mean of 22 mg CO(2)/dm(2).hour for 10 hexaploid genotypes. The decaploid genotype exhibited a ribulose 1,5-bisphosphate (RuBP) carboxylase specific activity 1.3- to 2-fold higher than typical tall fescue genotypes. Specific activities of photorespiratory enzymes and nitrate reduction enzymes were lower in the decaploid than the hexaploid genotypes. Results suggest that genetic expression of RuBP carboxylase activity may have been altered to increase the net photosynthesis rate in the decaploid genotype.