Trade is an important theme in President George W Bush's visit to Africa.
Africa could compete in the cotton market
Many countries have special access to the US market for many of their goods.
But they also want global talks underway in the World Trade Organisation (WTO) to make further reductions in the trade barriers they face.
Many African goods sold to the US face no tariffs - taxes which are applied only to imports.
An American law, called the Africa Growth and Opportunity Act, established this arrangement in 2000.
It is, for many African countries, a welcome improvement in their access to the world's largest economy.
But some of their most important products - in agriculture - still have to compete with subsidised American farmers.
There was great resentment in Africa last year when President Bush increased financial support for US farmers.
Africa could compete very effectively in producing cotton especially.
But US cotton farmers were among the beneficiaries of the increased support.
Farm subsidies are one of the main items in global negotiations conducted in the WTO.
But the talks have run into difficulty.
The US is not seen as the main obstacle to progress - the European Union and Japan appear to be the most reluctant to make deep cuts in subsidies and agricultural tariffs.
But as long as those talks fail to produce a result, US subsidies will remain in areas that make it harder for some African farmers to export what they produce.