Wildlife experts from India and Bangladesh are starting a census of Royal Bengal tigers in the Sundarbans, the world's largest mangrove forest.
The census is part of an initiative to preserve the biodiversity of the forest, which lies on the border between the two countries.
Zoologists estimate there are 5-600 tigers left in the Sundarbans, down from about 100,000 at the start of the last century.
It will be the world's largest census of tigers in a single geographical area.
The Sunderbans cover more than 10,000 square kilometres.
The census will count the tigers by collecting impressions of their paw-prints from the mangrove mud.
Wildlife officials say that previous counts were probably inaccurate, because some tigers were missed out and others were counted twice as they crossed from one country to the other.
At the last count last year, the Indian side of the forest contained 271 of the animals.
There has been no such count in Bangladesh in recent years.
Poaching and illegal logging have affected the tigers' habitat in the mangrove forest, reducing their number in Bangladesh in the past few years.
This is despite reports that some of the tigers from the Indian side have migrated to Bangladesh during this period.
The UN-funded project will involve not only the counting of Royal Bengal tigers, but also a study of their breeding and feeding behaviour.
Tigers kill nearly 50 people every year in the Indian Sunderbans alone.