The salamander-like axolotl apparently has not disappeared from its only known natural habitat in Mexico City's few remaining lakes.
Researchers say they have sighted, but not caught, two of the slippery little creatures during a second effort to find them.
A weeks-long effort last year by researchers in skiffs trying to net axolotls in the shallow, muddy waters of Xochimilco lake found none, raising fears that they might only now survive in captivity.
But biologist Armando Tovar Garza of Mexico's National Autonomous University said that members of the team carrying out the search had seen two axolotls during the first three weeks of a second survey expected to conclude in April.
"We weren't able to capture them ... because the behaviour of the axolotl makes them very difficult to capture," he said.
"We haven't had any captures, but we have had two sightings. That's important, because it tells us we still have a chance."
The axolotl, admittedly ugly with a slimy tail, plumage-like gills and mouth that curls into an odd smile, is known as the "water monster" and the "Mexican walking fish".
Its only natural habitat is the Xochimilco network of lakes and canals - but the "floating gardens" of earth piled on reed mats that the Aztecs built to grow crops but are now suffering from pollution, urban sprawl and invasive species.
The creature is import in scientific research because of its ability to regenerate severed limbs.
Some axolotls still survive in aquariums, water tanks and research labs, but experts said those conditions are not the best, because of interbreeding and other risks.
Releasing captive-bred axolotls into the wild could spread a fungus infection that is fatal to them and could reduce their genetic diversity.
Mr Tovar Garza said some small mutations, possibly the result of interbreeding, have already been seen.
Alarmed by the creature's falling numbers in recent years, researchers built axolotl "shelters" in Xochimilco to help them breed in the cleanest part of their remaining habitat.
Sacks of rocks and reedy plants act as filters around a selected area, and cleaner water is pumped in, to create better conditions.
The shelters also include permeable cages and other devices intended to help protect axolotls from non-native carp and tilapia that were introduced to the lake system years ago and compete with axolotls for food.
Growing up to a foot long, axolotls use four stubby legs to drag themselves along the bottom or thick tails to swim in Xochimilco's murky channels while feeding on aquatic insects, small fish and crustaceans.
But the surrounding garden-islands have increasingly been converted to illicit shantytowns, with untreated sewage often running off into the water.
The Mexican Academy of Sciences said a 1998 survey found an average of 6,000 axolotls per square kilometre, a figure that dropped to 1,000 in a 2003 study and 100 in 2008.