A plant was found in the C(3)-C(4) intermediate species, Flaveria linearis, in which apparent photosynthesis is stimulated by atmospheric O(2) concentrations. A survey of 44 selfed progeny of the plant showed that the O(2) stimulation of apparent photosynthesis was passed on to the progeny. When leaves equilibrated at 210 milliliters per liter O(2) were transferred to 20 milliliters per liter O(2) apparent photosynthesis was initially stimulated, but gradually declined so that at 30 to 40 minutes the rate was only about 80 to 85% of that at 210 milliliters per liter O(2). Switching from 20 to 210 milliliters per liter caused the opposite transition in apparent photosynthesis. All other plants of F. linearis reached steady rates within 5 minutes after switching O(2) that were 20 to 24% lower in 210 than in 20 milliliters per liter O(2). At low intercellular CO(2) concentrations and low irradiances, O(2) inhibition of apparent photosynthesis of the aberrant plant was similar to that in normal plants, but at an irradiance of 2 millimoles quanta per square meter per second and near 300 microliters per liter CO(2) apparent photosynthesis was consistently higher at 210 than at 20 milliliters per liter O(2). In morphology and leaf anatomy, the aberrant plant is like the normal plants in F. linearis. The stimulation of apparent photosynthesis at air levels of O(2) in the aberrant plant is similar to other literature reports on observations with C(3) plants at high CO(2) concentrations, high irradiance and/or low temperatures, and may be related to limitation of photosynthesis by triose phosphate utilization.