Red squirrels have found their way across the Menai Strait from Anglesey to Gwynedd, conservationists believe.
Two squirrels were found at Treborth and on the Faenol estate near Bangor - the first time a member of the species had been found in the area for decades.
Scientists are now waiting for the results of DNA testing to establish if they came from the Anglesey colonies.
Over 300 red squirrels live on Anglesey following 10 years of efforts to clear grey squirrels from the island.
Now Friends of Anglesey Red Squirrel think the endangered animals may have found their way across one of the Menai Strait bridges and started to colonise new habitats on the mainland.
Conservationist Dr Craig Shuttleworth thinks further culling of grey squirrels will be necessary to allow the newly discovered reds to thrive.
"The last squirrel recorded at Treborth was in 1976 so this is great news," he said.
"It would be nice to have a colony in Gwynedd.
"The squirrels were two young adult males, which are sort of teenagers - they are pioneering animals, risk takers.
"I suspect they came along the railway line, which has a gravel path and plenty of light and quiet.
"From a scientific point of view it quite clearly demonstrates they can get across, either by bridge or by swimming, which is less likely."
Reds are always under threat from greys because of competition for food and because the greys carry the squirrel pox virus which does not affect them but has a devastating effect on reds.
According to Friends of Anglesey Red Squirrels, the mammals were once widespread on Anglesey but the population declined sharply in the late 1980s as grey squirrels started to predominate.
Two years ago, residents on the island were encouraged to call a special phone line to report sightings of grey squirrels so they could be culled in a bid to completely eradicate greys from the island for good.