Wheat flag leaves having axial lesions caused by infection with Septoria nodorum were studied to determine how grain yield was affected and whether there was interference with translocation of carbohydrate. Kernel yield was reduced less than 5% when the axil and sheath of the flag leaf were the only tissues infected. When all leaves except the flag leaf were removed, kernel yield was reduced 14%, and when all leaves were removed, the reduction was 19%. After administration of glucose-U- 14C, translocation of 14C from the flag-leaf lamina to the developing kernels was not impeded; rather, in some instances, it was enhanced by infection with Septoria. In addition, glucose- 14C was metabolized more readily to ether-soluble components in infected tissue than in control tissue. Because axial lesions did not interfere with translocation of assimilates, the yield damage must have been caused by interference with other metabolic functions.