AbstractIn a population ofAtrichum undulatumwithn= 14, gametophytic interphase nuclei included two unequal, but substantial, blocks of heterochromatinsensu lato. Positive C-banding within each was limited to the terminal portion. Only one of these two chromosomes, together with its homologue, entered meiosis with a similarly extensive distribution of heterochromatin. The bivalent involved was metacentric and usually achiasmate in the heterochromatic region, which amounted to almost the whole of one arm. A single proximal chiasma was rare. In addition to this behavioural difference between the two basically haploid sets of seven chromosomes included in what has been regarded as auto diploidA. undulatum, evidence of a morphological distinction is presented. Six pairs were recognizable on morphological grounds but two chromosomes were unique. Meiosis in triploidA. undulatumwithn= 21 also included only a single heterochromatic bivalent.Variation between basically haploid complements is also presented forPhilonotis fontana, in a population of which a dimorphic bivalent was consistently present during meiosis. The dimorphism was related to the presence or absence of a short heterochromatic arm and, hence, of synapsis between telocentric and acrocentric homologues. Sampling to date suggests an association with dioecism.