Panicum milioides, a naturally occurring species with reduced photorespiration, P. bisulcatum, a C3 species, and P. miliaceum, a C4 species, were grown for 4 weeks at altered pO2 and pCO2 and several vegetative growth parameters were determined at weekly intervals. Compared to a pO2 of 10%, a greater O2 inhibition of the relative growth rate and dry matter production was observed for P. bisulcatum than for P. milioides at both 21% and 40% O2, whereas little effect of O2 was noted for P. miliaceum. Similarly, exposures to elevated pCO2 of 500 and 1000 μ1 CO2/liter resulted in a greater stimulation of vegetative growth for P. bisulcatum than for P. milioides, with little effect on P. miliaceum. The CO2 compensation concentration of P. milioides was less than that of P. bisulcatum over a pO2 range of 5 to 40%. At 5% O2, the compensation concentration was relatively O2-insensitive, whereas above 5% it increased with increasing pO2. It is concluded that P. milioides represents the first well documented example of a C3 plant with reduced photorespiration, based on both leaf CO2 exchange parameters and growth analyses of dry matter production.