The ban was hailed by the United Nations Environment Programme as a breakthrough.
“Our oceans are being turned into rubbish dumps,” says Erik Solheim, the head of the agency.
As Kenyans get richer and move to cities, the amount of plastic they use is growing.
By one estimate, Kenya gets through 24m bags a month, or two per person. (Americans, by comparison, use roughly three per person. )
Between 2010 and 2014 annual plastic production in Kenya expanded by a third, to 400,000 tonnes.
Bags made up a large part of the growth.
Kenya has tried to ban polythene bags twice before, in 2007 and 2011, without much success.
This latest measure is broader, but few are ready for it.
The Kenyan Association of Manufacturers says it will cost thousands of jobs.
Some worry that supermarkets will simply switch to paper bags, which could add to deforestation.
And then there is the question of whether Kenyan consumers will accept it.
In Rwanda, since its ban was imposed, a thriving underground industry has emerged smuggling the bags from neighbouring Congo.
Packing in the plastic may be harder than it seems.