Genetic modification of Rubisco to increase the specificity for CO2 relative to O2 (τ) would decrease photorespiration and in principle should increase crop productivity. When the kinetic properties of Rubisco from different photosynthetic organisms are compared, it appears that forms with high τ have low maximum catalytic rates of carboxylation per active site (kcc). If it is assumed that an inverse relationship between kcc and τ exists, as implied from measurements, and that an increased concentration of Rubisco per unit leaf area is not possible, will increasing τ result in increased leaf and canopy photosynthesis? A steady-state biochemical model for leaf photosynthesis was coupled to a canopy biophysical microclimate model and used to explore this question. C3 photosynthetic CO2 uptake rate (A) is either limited by the maximum rate of Rubisco activity (Vcmax) or by the rate of regeneration of ribulose-1,5-bisphosphate, in turn determined by the rate of whole chain electron transport (J). Thus, if J is limiting, an increase in τ will increase net CO2 uptake because more products of the electron transport chain will be partitioned away from photorespiration into photosynthesis. The effect of an increase in τ on Rubisco-limited photosynthesis depends on both kcc and the concentration of CO2 ([CO2]). Assuming a strict inverse relationship between kcc and τ, the simulations showed that a decrease, not an increase, in τ increases Rubisco-limited photosynthesis at the current atmospheric [CO2], but the increase is observed only in high light. In crop canopies, significant amounts of both light-limited and light-saturated photosynthesis contribute to total crop carbon gain. For canopies, the present average τ found in C3 terrestrial plants is supra-optimal for the present atmospheric [CO2] of 370 µmol mol−1, but would be optimal for a CO2 concentration of around 200 µmol mol−1, a value close to the average of the last 400 000 years. Replacing the average Rubisco of terrestrial C3 plants with one having a lower and optimal τ would increase canopy carbon gain by 3%. Because there are significant deviations from the strict inverse relationship between kcc and τ, the canopy model was also used to compare the rates of canopy photosynthesis for several Rubiscos with well-defined kinetic constants. These simulations suggest that very substantial increases (> 25%) in crop carbon gain could result if specific Rubiscos having either a higher τ or higher kcc were successfully expressed in C3 plants.