Hexaploid Triticum crassum Aitch &Hemsl. (syn. Aegilops crassa Boiss.) and Triticum syriacum Bowden [syn. Aegilops crassa Boiss. var. palaestina Eig, Aegilops vavilovii (Zhuk.) Chen.] are members of the T. crassum complex. Previous studies suggested that they have the D genomes of T. tauschii (Coss.) Schmalh., but the sources of the other genomes of these species remain uncertain. In the present investigation, variation in 27 repeated nucleotide sequences and the 5S rRNA loci was used to determine the genome origin of these hexaploid species. This study indicated that the two hexaploid species share two pairs of genomes that were contributed by a common ancestral tetraploid with a genome formula DcDcXX. It was concluded that the Dc genome was contributed by ancient T. tauschii and the X genome by an extinct species, possibly a species that was ancestral to the entire genus Triticum L. sensu Bowden except for T. tauschii. The third genome of hexaploid T. crassum is virtually identical to the D genome of extant T. tauschii and that of T. syriacum is identical to the Sse genome of T. searsii (syn. Aegilops searsii Feldman et Kislev). The genome formula of hexaploid T. crassum was proposed to be DcDcXXDD and that of T. syriacum DcDcXXSseSse to reflect the results reported here. The present study suggested that the apparent modification of the Dc and X genomes of the hexaploids resulted from evolutionary divergence.Key words: Triticum, Aegilops, Aegilops crassa, Aegilops vavilovii, phylogeny.