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The present study on the genus Brachypodium has been carried out from a biosystematic view-point. The following taxa were used: B. sylvaticum, B. glaucovirens, B. pinnatum, B. rupestre, B. phoenicoides, B. retusum, B. mexicanum and B. distachyon. All the species are perennial and chasmogamous except B. distachyon, which is annual and only partially chasmogamous. B. sylvaticum, which is the most widespread species of the genus, overlaps most of its characters with those of B. pinnatum and B. glaucovirens. This and other taxonomic problems were investigated by the study of morphology, anatomy, cytology, seed-protein band analysis and artificial hybridisation. The main morphological variations which were analysed are the pubescence of plants; leaf-blade length, width and colour; leaf-blade rib shape; and lemma awn length. Special attention was paid to wild-collected plants intermediate in appearance between various pairs of species. Species of the genus Brachypodium have been artificially hybridized in this study for the first time; most crosses were successful in producing F hybrids. The degree of success was found to be a valuable new taxonomic character in the genus. Natural hybridization was shown to occur in some cases, but seems to be very rare. A complex situation of intraspecific chromosome number variation exists in the genus. Chromosome counts with their locality and country of origin are given in Appendix 1. B. pinnatum with 2n=36 was found for the first time. The evolution of chromosome numbers in the genus is discussed; the basic chromosome number seems to be 9. Meiosis is regular in the species but in some wild intermediates it is slightly irregular. Chiasma frequency is highest in B. sylvaticum and B. glaucovirens and lowest in B. phoenicoides. Synthetic hybrids could be divided into three groups according to their meiotic behaviour, from slightly irregular or almost regular to very irregular. The evolution of Brachypodium and a summary classification of the representatives used in this study is presented in the Discussion. It is concluded that B. distachyon should remain in the genus Brachypodium in a separate section from all the other species. Brachypodium sylvaticum, B. glaucovirens, B. pinnatum, B. phoenicoides, B. retusum, and B. mexicanum are to be considered to be separate species, but B. rupestre should be relegated to varietal rank under B. pinnatum.