It is widely agreed that as temperatures rise above 30◦C, the impairment of photosynthetic activity at current atmospheric CO2 concentrations is in large part the resul to flower Rca activity or its interaction with Rubisco . Effects of altered Rca activity on non-steady state photosynthesis have been demonstrated but sustained faster rates of carbon assimilation seem to be more subtle and leave the question of long-term effects on plant growth in hot environments unresolved. In fact, many studies show limited direct evidence of leaf-level, steady-state photosynthesis impacting on yield. In this light,aheat-tolerantform of Rca from O. australiensis was expressed in O. sativa and plant phenotype was tested over the life of the plant with a particular view to assessing yield. We demonstrate a dramatic improvement in growth and yield of heat stressed rice associated with subtle improvements in photosynthetic potential.