AbstractSince 1794 about 1.3 species per year have been added to the known bryophyte flora of Oxfordshire.This is chiefly due to taxonomic research combined with diligent and intelligent collecting but it is also partly due to the immigration or at least the increasing frequency of some species. Twenty- five species (ca7% of the flora) have not been recorded during the present century, and may be extinct, but many more have declined in frequency. The paper seeks to elucidate the extent and the causes of such changes; for this purpose much evidence from other districts is reviewed.Apart from the physical destruction of local specialized habitats and the creation of new habitats, the more important causes of widespread change have been: (a) eutrophication of both land and water, partly as a result of (b) changes in agricultural practice, including the heavy application of fertilizers, and immediate ploughing and resowing after harvest; (c) a decline in grazing, especially by rabbits; (d) changes in forestry practice, including the cessation of coppicing and the planting of conifers, increasing the amount of decaying woody material; (e) most far-reaching of all, widespread but slight atmospheric pollution, the effect of which seems to be due to‘acid rain’rather than to toxic SO2.Factors (d) and (e), though adverse for basiphil species, have favoured acidiphil species, at least eight of which have increased in frequency. An additional four acidiphil species first recorded in the county since 1935 have remained rare, but evidence from other districts suggests that they are increasing.Some dioecious mosses formerly fruited more freely in Oxfordshire than they do now.