Zanzibar's ban on the import and production of plastic bags has come into effect.
Tourism is the backbone of Zanzibar's economy
Authorities on the semi-autonomous Tanzanian islands say discarded bags damage the marine environment and hurt its crucial tourism industry.
The government, which approved the law in July, has suggested the use of raffia bags as an alternative.
But the BBC's Ally Saleh in Zanzibar says many people are sceptical about whether the ban will be enforced.
Tourism is the backbone of Zanzibar's economy, outstripping the trade in spices, traditionally the island's biggest foreign currency earner.
Our correspondent says the archipelago's streets are littered with rubbish as municipal authorities are only able to collect about a third of the waste produced daily.
"We have to put the environment above everything," Zanzibar's Director of Environment Ali Juma said.
"Besides being an eyesore, plastic bags are very damaging to land and marine life and we are already threatened by the rapid pace of development."
He said that anyone violating the ban risked a jail sentence of up to six months or a fine of $2,000 or both punishments.
Some people are worried about the loss of tax revenue to the islands.
It is estimated more than 200 tonnes of bags pass through Zanzibar's port every month destined for local and regional markets.
"While it is environmentally sound to ban the plastic bags, the government is going to lose $400,000 per month as revenue for bags on transit and destined for the local market," businessman Salim Turky told Reuters news agency.
Rwanda has also banned the use of plastic bags for environmental reasons.