Chinese scientists will continue to search for a rare freshwater dolphin unique to the Yangtze River after a 38-day search failed to find any, Xinhua news agency said.
Foreign experts have already concluded that the baiji, or white-flag dolphin, is extinct, becoming a victim of development, overfishing and shipping along China's longest river.
But Wang Ding, head of a team of scientists that concluded their search for the baiji last week, said the efforts to search for and protect the dolphin should continue as there might be some of the mammals left in the wild.
The long-beaked, nearly blind baiji is related to other freshwater dolphin species found in the Mekong, Indus, Ganges and Amazon rivers.
In the late 1970s, scientists believed several hundred baiji were still alive, but by 1997 a survey listed just 13 sightings. The last confirmed sighting was in 2004, and the last captive baiji died in 2002.
The Chinese government had set up a reserve in a lake in central Hubei province to look after captured baiji but failed to find any.
The six-week expedition - made up of two ships and 30 scientists from Japan, China, the United States and Switzerland - did spot about 300 of another threatened species, the Yangtze finless porpoise, far less than they had thought they would see.