The chromosome number, karyotype, and morphological variation of plants of the Deschampsia caespitosa complex are reported in this paper. It is shown that the chromosome number of 2n = 26 is most common in typical D. caespitosa (L.) Beauv. But, remembering the basic chromosome number in the Agrostideae, the original basic number of the genus Deschampsia is considered to be x = 7. Thus, a reduction of the chromosome number from 28 to 26 is speculated in this particular group of Deschampsia. The plant from Mt. Albert, Canada, referable to D. caespitosa ssp. littoralis (Reut.) Rchb., has a high chromosome number of 2n = 49. Considering the morphological affinity between D. caespitosa ssp. littoralis and D. alpina Roem. & Schult., as well as their geographical distribution, parallel development of the plants with such high irregular chromosome number is assumed.The karyotype of D. caespitosa s. str. was also investigated. This species has a highly specialized asymmetrical karyotype, viz., K(2n) = 26 = 4 V + 10 J + 12 I.On the basis of cytology, morphology, ecology, and geographical distribution, the phylogenetic relationships within this complex were considered. The littoralis type, having well-developed panicles, large spikelets, three (rarely four) florets, and a preference for wet terrestrial sites, is speculated to be the most basic form in the D. caespitosa complex, even though the type with smaller spikelets, i.e., caespitosa s. str. is at the present time much more abundant. Various other population groups such as D. caespitosa v. parviflora (Thuill.) Coss. & Germ., D. brevifolia R. Br., D. alpina Roem. & Schult., and the species of the bottnica group (including D. beringensis Hultén, D. bottnica (Wahlenb.) Trin., D. congestiformis Booth, D. holciformis F. & C. Presl, D. mackenzieana Raup, D. macrothyrsa Tatewaki et Ohwi, D. obensis Roshev., and D. wibeliana Schur.) are presumed to be more specialized.