Speltoid mutants, which have slender spikes, were found in the offspring of a cross, euploid common wheat, Triticum aestivum cv. ''Chinese Spring'' (.female.) .times. an alien addition line of Chinese Spring having the long arm of an Aegilops longissima chromosome. With the offspring of three of these speltoid mutants, chromosome mapping was conducted for a speltoid suppression gene Q of common wheat. The C-banding study demonstrated that the spletoid character was always associated with partial deletions in the long arm of chromosome 5A. The abnormal 5A with the least deletion retained the intercalary C-band which was located at about 54 per cent of the long arm from the centromere. Therefore, it was properly decided that the Q gene was situated somewhere in the distal 46 per cent region of the 5A long arm.