A model of leaf, photosynthesis has been developed for C3-C4 intermediate species found in the generaPanicum, Moricandia, Parthenium andMollugo where no functional C4 pathway has been identified. Model assumptions are a functional C3 cycle in both mesophyll and bundle-sheath cells and that glycine formed in the mesophyll, as a consequence of the oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39), diffuses to the bundle sheath, where most of the photorespiratory CO2 is released. The model describes the observed gas-exchange characteristics of these C3-C4 intermediates, such as low CO2-compensation points (Γ) at an O2 pressure of 200 mbar, a curvilinear response of Γ to changing O2 pressures, and typical responses of CO2-assimilation rate to intercellular CO2 pressure. The model predicts that bundle-sheath CO2 concentration is highest at low mesophyll CO2 pressures and decreases as mesophyll CO2 pressure increases. A partitioning of 5-15% of the total leaf Rubisco into the bundle-sheath cells and a bundlesheath conductance similar to that proposed for C4 species best mimics the gas-exchange results. The model predicts C3-like carbon-isotope discrimination for photosynthesis at atmospheric levels of CO2, but at low CO2 pressures it predicts a higher discrimination than is typically found during C3 photosynthesis at lower CO2 pressures.