AbstractApplication of a Giemsa C-banding technique to the three British species of Pellia indicates that the original description of heterochromatin was largely if not entirely based on constitutive rather than facultative heterochromatin. The technique is shown to have considerable potential in defining liverwort karyotypes more precisely than has hitherto been·possible and to assist in the cytological comparison of species. In this respect, the similarity between specimens ofP. epiphyllaandP. neesianaappears to be greater than the resemblance of either of these species toP. endiviifolia. Chromosome inversion is reported for the first time in bryophytes, inP. neesiana, but there is strong evidence to suggest that structural rearrangement, though not necessarily inversion, has also occurred elsewhere in--this genus.Unequal, almost totally euchromatic sex-specific chromosomes are described for a specimen of BritishRiccardia pinguis. Similarities between the Giemsa C-band patterns of this species andCryptothallus mirabiliswere not as great as might have been expected. Evidence is presented of unusually early duplication of chromatids in the latter.The reaction of these five species to the Giemsa C-banding technique reveals a number of difficulties and a lack of precision associated with the terms H- and h-chromosomes. It is therefore recommended that their use in liverwort cytology should be discontinued.