Knowledge of the distribution of arable bryophytes lags behind that of bryophytes of many other habitats, and we have almost no information on their occurrence in relation to crop type and agricultural management regime. A survey of arable fields in Britain and Ireland carried out by members of the British Bryological Society in 2001–05 was designed to provide such baseline data. We surveyed a stratified random sample of 200 fields in the main areas of arable agriculture and a further 620 fields in these areas and in areas where arable fields are less frequent. The species present in each field were listed and their frequency within the field assessed. Data on the crop, the cover of vascular plants, bryophytes, trash and bare soil, and the pH and texture of the soil were recorded for each field. Six species assemblages are described on the basis of a classification of the species recorded in each field and their frequency values. The Tortula truncata–Anthoceros and Dicranella staphylina–Riccia glauca assemblages are the most species-rich and are concentrated on acidic soils in northern and western Britain and in Ireland. By contrast, the Barbula unguiculata–Bryum klinggraeffii and Phascum cuspidatum–Microbryum davallianum assemblages are mainly found on calcareous soils in southern and eastern England. The Bryum dichotomum–Marchantia polymorpha assemblage is characterised by widespread, weedy generalist species and is concentrated in East Anglia. The final assemblage, Brachythecium rutabulum–Fissidens taxifolius, characteristically occurs in late-successional fields and was scattered throughout the area sampled. A comparison of the results from Kent with those of a survey of Kentish fields (using a different methodology) by A.G. Side in 1973–74 suggests that the Bryum dichotomum–Marchantia polymorpha assemblage may possibly have replaced the Dicranella staphylina–Riccia glauca assemblage on acidic soils in the last 30 years. The considerable geographical variation within the arable flora of Britain and Ireland revealed by the survey should be taken into account when assessing the meagre evidence for historic changes in the arable bryophyte flora.