Ivory Coast's cocoa farmers have gone on strike to protest at low prices in the world's biggest producer.
Cocoa has kept Ivory Coast going through the four-year conflict
Farmers in the trade union Anaproci, which represents 80% of Ivory Coast's 700,000 cocoa farmers, are preventing cocoa from getting to ports.
Some 7m Ivorians, of a population of 18m depend on money from the raw ingredient of chocolate.
The strike follows a warning over the spread of a cocoa disease.
Cocoa lies at the heart of what was one of West Africa's strongest economies, before the country was divided by civil war in 2002.
"The warehouses are open in San Pedro but they are empty. There are no lorries in front of the warehouses. Hussein Attie," a cocoa buyer in western San Pedro port, told Reuters news agency.
"The strike has started and the effects are already visible."
Ivory Coast's Coffee and Cocoa Exchange has set an indicative price of 400 CFA francs (80 US cents), for 1kg of cocoa.
But the BBC's James Copnall in Abidjan says farmers rarely receive this amount - in part because the exporters who buy the cocoa pay a high tax to take it out of the country.
The cocoa farmers went on strike in 1999 and 2004 and succeeded in blocking cocoa from getting to the ports.
Ivory Coast produces about 40% of the world's cocoa.
Our reporter says money from cocoa and coffee and latterly from oil has enabled the Ivorian government to keep afloat, even though the country has been divided in two by civil war for the last four years.
Last week, Ghanaian researcher H Dzahini-Obiatey told a gathering of cocoa experts that Ghana and Ivory Coast were struggling to control the swollen shoot virus.
The virus, which defoliates and can eventually kill cocoa trees, has been damaging yields in West Africa.