Most upland rice cultivars grow successfully in flooded conditions, whereas most lowland rice cultivars can survive in well-watered aerobic soils. It is known that rice has C3 photosynthesis, whereas the weed Echinochloa oryzicola possesses a C4 photosynthetic pathway. It was hypothesized that plant species with a C4 photosynthetic pathway had a greater growth rate than C3 plants (Black 1971). Scientists such as Baskin and Baskin (1978) reviewed several studies and obtained results that generally disagreed with the hypothesis. They suggested that many factors other than the C4 pathway are more important in determining plant growth rate. Little is known about the comparative growth performance of rice in upland and lowland field conditions with E. oryzicola as a "vegetative mimic" of rice. This study compared the growth performance of rice and E. oryzicola under upland and lowland conditions and determined related photosynthetic traits such as stomatal conductance and net photosynthetic rate.