An experimental taxonomic study, of a group of closely related species of the moss genus Dicranum, has been carried out. Morphologically the material studied falls into four major taxa–D. bonjeanii, D. fuscescens, D. majus and D. scoparium. Cytologically three of the taxa (only a single chromosome count is available for D. bonjeanii, n = 11) show variation both in the number of ‘A’ chromosomes and in the number of minute bivalents: D. fuscescens, n = 10, 11 (10 + 1 m), 12, 24; D. majus, n = 11, 12, 13 (12 + 2 – 5 m)’D. scoparium, n = 11, 12, 13 (12 + 1 m). These chromosome numbers suggest that each taxon is an aggregate of closely related species, but until more evidence is available it is recommended that four taxa of specific rank be recognized. This procedure receives support from the fact that the different cytotypes are indistinguishable or have not yet received thorough taxonomic study. The intraspecific variation within each of the four species has been examined. Cultivation experiments have shown that certain genotypes exhibit wide phenotypic plasticity, but distinct genotypes of D. fuscescens and D. scoparium are reported. The use of varietal names for these morphologically distinct populations is not recommended, as they cannot be distinguished (except by cultivation experiments) from genotypes which are phenotypically plastic.