AbstractCytological interpretation of bryophyte phylogeny is closely concerned with reproductive behaviour. The development of structural sex specific chromosomes differing only in that the Y includes more heterochromatinsensu latothan the X is indicative of monoecism having preceded dioecism in mosses (Bryophyta or Musci) and liverworts (Hepatophyta) at the lowest level of chromosome number now known. Insufficient data preclude similar analysis of the Anthocerotophyta. Evolution within the Musci has involved a significant degree of polyploidy as well as aneuploidy, with dioecy having arisen again at the level ofn= 2x, the effects of which are seen in the less widespread evidence of subsequent polyploidy. Numerical change among the chromosomes of the Hepatophyta is almost completely confined to very limited polyploidy and is attributed to the early establishment of dioecy at the level ofn= 8, 9 and 10, infrequent sexual reproduction and extensive clonal development. C-, Q-, N- and Hoechst 33258-banding, however, suggest that evolution within liverworts is continuing in hitherto unsuspected ways. Thus, there are pronounced cytological differences betweenPlagiochila porelloides(n= 9) andP. asplenioides(n= 9) and considerable molecular differences between the constitutive heterochromatin ofPellia epiphylla(n= 9) andP. neesiana(n= 8+X/N). It is also suggested that neitherPlagiochila britannica(n= 18) norPellia borealis(n= 18) is an autodiploid of recent origin and that at least the former may be an allopolyploid. All these findings are discussed in the context of related groups of green algae and of pteridophytes.