AbstractThe karyology ofCarex flavaaggregate in Switzerland was investigated. Fertility was measured and crossings were done.The microsporogenesis is described.In Switzerland, theC. flavagroup consists ofC. flavavar.flava(n = 30, 31),C. flavavar.alpina(n = 30, 31),C. lepidocarpa(n = 34),C. demissa(n = 34, 35), andC. serotina(n = 34, 35, 36). The three species with high chromosome numbers often grow together withC. flava. Accordingly, F1‐hybrids with this parental species are frequent. Their meiosis and fertility are profoundly disturbed. With artificial hybrids, disturbances decrease as the differences in chromosome numbers of the parents decrease. Backcrossings with the parents over many generations lead to forms identical in morphology and fertility to the pure species, which are called stabilized cryptic backcrosses. The extensive genetic and karyotype‐variability within populations, particularly ofC. flava, might be due to this introgression. In other parts of Europe (e.g. Britain) hybrids withC. flavaas one parent are rare. As a consequence, introgression plays a little role and karyotypes are stable.Karyotype evolution inC. flavagroup is quite different from karyotype evolution in otherCarexspecies. This is due to differences in life history tact