Examples of both the 410 and 500 bp size classes of repeating units containing wheat 5S rRNA genes have been cloned in plasmid pBR322 and sequenced. The structural genes showed sequence microheterogeneity. Also the gene in the 500 bp repeat which was sequenced had a 15 bp tandem duplication within it and appears to be representative of non-transcribed subfamily of repeating units. The transcription terminators comprise 14-17 A.T bp immediately preceded by a region of weak dyad symmetry. The spacer regions adjacent to the transcription terminators in the two different size repeat units have interspersed oligonucleotides of high and low homology. The central spacer regions of the two size classes have very different sequences. The only repeated sequence in the spacers has undergone extensive divergence. In contrast to most of the spacer, the 70 bp region preceding the genes of each type of repeat show high homology, suggesting that it has functional importance. The transcription start point obeys the pyrimidine-1 purine+1 rule.