PEOPLE - One out of four black Africans live in Nigeria, making the country the most populous on the continent. Nigeria's population density is about double that of Africa as a whole. About 20 per cent of Nigeria's estimated 80 million to 100 million population live in urban areas.

Nigeria's 250 ethnic groups give the country a wide variety of customs, languages, and traditions. The northern two-thirds of Nigeria is dominated by the Hausa tribe, which is mainly Moslem. Smaller ethnic groups in the north include the Fulani, Nupe, Tiv and Kanuri. The major ethnic group in the southwest is the Yoruba tribe, which is largely made up of Christians. Ibo tribesmen, who traditionally have wielded great influence in trade, the professions and the bureaucracy, predominate in the southeast where they tried unsuccessfully to form a breakaway republic in the late 1960s.

About 47 percent of Nigerians are Moslems, 34 percent are Christians and 19 percent practice animist or other faiths.

LAND - The country has four main topographical areas: a hot, humid coastal belt of mangrove swamp, a tropical rain forest and oil palm bush north of this belt, a high central plateau of woodland and savanna over much of the north and semidesert in the far north.

HISTORY - Nigeria formerly consisted of a number of areas under separate British colonial administrations. In 1954 Nigeria became a federation under a governor-general and then received its independence Oct. 1, 1960.

From the start Nigeria faced tribal and regional tensions, notably between the south and the north. This culminated in the mid 1960s in an attempt by the southern Ibos to secede.

In 1975, a bloodless coup installed Gen. Murtala Muhammed in power. He was assassinated in February, 1976, in an abortive coup plot, and the government replaced him with the armed forces chief of staff, Lt. Gen. Olusegun Obasanjo.

GOVERNMENT - Nigeria is currently ruled by a 23-man Supreme Military Council combining executive and legislative powers. The Federal Executive Council includes both civilian and military members and acts as a Cabinet. Lt. Gen. Obasanjo heads both bodies. All 19 states now have military governors. While the states have their own budgets, they are heavily dependent on the federal government for much of their revenues.

ECONOMY - The Nigeria economy revolves around annual oil revenues of more than $9 billion, a figure that should jump this year because of the latest series of oil price increases by the Organization of Petroleum Exporting Countries, of which Nigeria is a member. CAPTION: Map, Shaded area shows where Ibos rebelled, By Dave Cook - The Washington Post