The contribution made by ear photosynthesis to grain yield was found to vary from 10 per cent to 44 per cent depending on the technique used, and on environmental conditions. A modified ear-shading technique is described which overcomes some of the undesirable features of previous ear-shading methods. It was also found that ear photosynthesis comprises two processes, (a) the assimilation of atmospheric CO2 and (b) the photosynthetic refixation of the ear's respiratory CO2. Dry-weight data and measurements of CO2 exchange both indicated that this latter component can make a significant contribution to grain yield.