Botswana has been ranked the least corrupt African country in a list compiled by the World Economic Forum (WEF).
Tunisia, Gambia and South Africa followed Botswana as the countries with the best standards of governance.
Nigeria and Chad were named as having the worst public institutions.
"The index is a very important component of our assessment of a country's competitiveness and overall prospects for economic growth", WEF economist Fiona Paua said.
The Swiss-based organisation graded 21 countries on the rule of law, impressions of corruption and the enforcement of contracts.
The WEF draws its evaluations from a survey of business leaders in the countries concerned.
This year WEF received about 2,000 responses for the survey.
The survey looks at law enforcement and corruption, combining the two to give an overall ranking on good governance.
South Africa was dragged down in law enforcement
because of its high rate of organised crime.
Zimbabwe, which is facing a political and
economic crisis, was ranked 16 and judged to have the least independent judiciary.
All the countries where the survey was carried out are partners of the WEF's global competitiveness programme.
The rankings were announced at the opening of the WEF's Africa economic summit in the South African city of Durban.
About 650 African parliamentarians and international business leaders are meeting there to promote investment and development in the world's poorest continent.
The rankings were released at a time when the European parliament has been holding hearings into payments made by international oil companies to African governments.
The hearings are part of a campaign to bring in a law requiring companies to reveal the money they pay governments for operating in their countries.
The European Parliament's spokesman on corporate responsibility, Richard Howitt, said companies were under pressure from corrupt governments not to reveal how much they pay in taxes and licensing fees, so that the money can be siphoned off for personal enrichment.