Nuclear DNA content of pollen tetrads of all species of the genus Aegilops [A. umbellulata, A. ovata, A. columnaris, A. biuncialis, A. triaristata, A. variabilis, A. kotschyi, A. triuncialis, A. caudata, A. cylindrica, A. comosa, A. uniaristata, A. mutica, A. speltoides, A. longissima, A. bicornis, A. squarrosa, A. ventricosa, A. crassa, A. vavilovii, A. juvenalis] were measured microspectrophotometrically. Nine diploid species have a species specific amount of DNA and are divided into a lower or a higher DNA group based on nuclear DNA content. The speciation in Aegilops diploids is associated with a change in DNA content, but not with a change of chromosome number. Significant differences were found among 3 varieties of A. squarrosa [var. typica, var. meyeri, var. strangulata]. Two tetraploids (A. triuncialis and A. cylindrica) whose parentage has been confirmed were comparable to the sum of parents in DNA content of the nucleus. No difference between 2 ssp. of A. triuncialis was found, but there was significant difference between A. variabilis and A. kotschyi which have the same genome constitution. In polyploid species whose diploid ancestors have not been clearly identified, because of modification of the genomes involved, some showed no change, some reduction and still others an increase in DNA content of the nucleus as compared with the putative ancestors.