Significant demographic disparities exist between Roman Catholics and Protestants in Northern Ireland. The larger size of Roman Catholic families is not a product of low Protestant fertility but, on the contrary, of a very high Roman Catholic birth rate. The traditional Irish custom of delaying age at marriage is still an important factor limiting the size of Roman Catholic families, although birth control is becoming increasingly accepted as Roman Catholic attitudes change. The fertility disparity between the two communities has traditionally been offset by a higher Roman Catholic rate of emi- gration, and although this is still the case, the differential is narrowing and the proportion of Roman Catholics in the total population has risen substantially during the last 25 years. If present trends are maintained, a Catholic majority is indicated sometime during the first half of the next century, and there is thus substance in the Protestant fear of being overtaken in the 'demographic numbers race'. Demographic differentials therefore aggravate inter-community distrust in the Province. The higher rate of Catholic emigration not only disrupts family ties, but lends superficial credence to the charge of discrimi- nation on the ground of religion, as does a higher rate of unemployment. That these adverse features may have some asso- ciation with high birth rates and large families tends to be overlooked; after all the population of the Irish Republic suffer from similar problems. IT IS clear that the present conflict in Northern Ireland is between two communities who feel nationally distinct from one another. While some view the conflict in terms of deciding who will control the geographical area of Northern Ireland, others still see it as the endeavour of the Roman Catholic minority to achieve the 'civil rights' that have long been denied them by the Protestant majority. Both sides have developed their own conventional wisdoms and mythologies founded on their own particular views of the political, historical and economic nature of Northern Ireland. Little, however, is known of the demographic background to the problem, which is rather sur- prising since differences of a demographic nature form important elements of the conflict. If the Roman Catholic population were a small and declining minority as are Protestants in the Irish Republic, the extremes of nationalism in Ulster would in all likelihood be a more manage- able phenomenon. The widespread belief that Roman Catholics will form an ever increasing proportion of the total population of the Province because of their higher rate of natural increase must feed the aspirations of the minority for a united Ireland, while at the same time making Protestants fearful for the future. Moreover, arguments can be put forward to support the view that Roman Catholics are disadvantaged in the fields of employment and housing not so much as a result of discrimination but because of their larger family sizes. A lower birth rate would also mean a lower rate of emigration and so help prevent the social disruption caused by the breakup and uprooting of families. The aim of this paper is therefore twofold: the major objective is to discuss and analyse the demographic differences between the two communities; the other is to discuss briefly the implications of some of these disparities in the economic, social and health fields. The 196I and 1971 censuses of population for Northern Ireland and the annual reports of the Registrar General provide the main sources of data for the study.