5S rRNA sequences from several accessions of Kengyilia alatavica, a member of a tribe that includes wheat and wheat relatives, have been amplified by the polymerase chain reaction, cloned, and sequenced. From an evaluation of the aligned sequences, five 5S unit classes have been discerned. One class consists of short units, while the other four contain longer units. BLAST searches of the GenBank database have allowed us to tentatively assign these to classes found in genomes of other species. For example, the short 5S unit class and one long 5S unit class were designated, respectively, "short P1" and "long P1" because of their match with the comparable sequenced 5S rDNA accessions of Agropyron cristatum, a carrier of the P genome. Another unit class, is coined as "long R1", because of its similarity to the units sequenced from Secale cereale and Secale vavilovii, carriers of the R genome. The third unit class was designated "long S1" and is found also in Elytrigia spicata, a carrier of the S genome. Implications of these findings on the possible association of some unit classes with cytological haplome and on concerted evolution are discussed.