The genus Chaenactis is represented by annual, biennial and perennial taxa that occupy relatively dry habitats in the western United States and adjacent Canada and Mexico. Morphological variation is considerable, and differences between certain taxa are often blurred, especially in regions where ranges overlap. Cronquist (1955), Munz (1959) and Ferris (Abrams and Ferris 1960) consider Chaenactis to include about 24 or 25 species, but Stockwell (1940) in his revision of the genus recognized 33 species. The monotypic Chamaechaenactis Rydb. and Orochaenactis Cov. have been erected for, respectively, C. scaposa Eastw. and C. thysanocarpha A. Gray. Stockwell (1940) reported chromosome counts of n = 5, 6, 7, or 8 in different species of Chaenactis, and found natural interspecific hybrids between some with different numbers of chromosomes. The existence of extensive morphological and cytological variation, haziness of specific distinctions, occurrence of natural interspecific hybrids and the relative ease with which plants are cultured and examined cytologically suggest that Chaenactis offers interesting possibilities for experimental studies. The purpose of this paper is to report some of the karyological information discovered in a continuing biosystematic study of Chaenactis and related taxa. The bulk of the work has been on the polymorphic and widely distributed Chaenactis douglasii complex, but I have also obtained chromosome counts from 12 other Chaenactis species and from Chamaechaenactis scaposa. As far as known, my counts for six of these species are first reports, as are the counts of 2n = 13, 14, 15, 15-17, 18, 25, 26, 27, 28 and ca 38 in the C. douglasii complex. This means that published counts are now available for all but two of the undoubtedly valid species of Chaenactis, thus permitting a better understanding of this interesting genus. Most of the chromosome counts were obtained by squashing microsporocytes of buds collected in the field or experimental garden in acetocarmine or propionocarmine. Young heads were fixed either in 1:3 acetic-alcohol or in 1:3:6 aceticchloroform-alcohol. The latter appeared to afford better fixation. The best preparations of wild plant material generally were obtained from plants in early bloom, provided that the heads were fixed before noon or after 5 p.m. Early-season bud material exhibited stringy, "sticky" and diffuse chromosomes, and buds collected during the warmest part of the day usually showed badly clumped chromosomes. A few counts were derived from propionocarmine preparations of radicles or root tips of garden-grown material fixed in 1: 3 acetic-alcohol, sometimes preceded by pretreatment with oxyquinolene. One count was obtained from tapetal cells. Table 1 shows the chromosome counts obtained by examining microsporocytes from one or more populations of 13 species of Chaenactis and from Chamraechaenactis scaposa. The counts for Chaenactis alpina, C. cusickii, C. evermannii, C. nevii,