A PCR system was designed to amplify 5S spacer rDNA specifically from homeologous chromosome 1 in a variety of species representative of the Aegilops and Triticum genera. Two polymerase chain reaction (PCR) primer combinations were used, one of which appears to be apomorphic in nature and specific to chromosome 1A in Triticum urartu and tetraploid and hexaploid wheats containing the AA genome donated by T. urartu. The value of studying single repeat types to investigate the molecular evolution of 5S-rDNA arrays is considered.