Endangered crayfish in the Bristol Avon have been moved to a new home in the Mendips, Somerset, in order to protect them from "aggressive alien species".
It is hoped the species will now be protected from the crayfish plague
Several hundred of the protected native white-clawed crayfish have been moved away from the American signal crayfish in the hope they will now flourish.
The move was made by the Environment Agency and the Avon Wildlife Trust.
The crayfish were caught from a threatened tributary in the By Brook catchment area near Chippenham.
The alien invaders were originally imported for food, but escaped into the wild where they now out-compete the smaller and less aggressive native white-clawed species.
"Non-native crayfish are bad news. Not only do they push out our native crayfish, they can carry the deadly crayfish plague which is responsible for wiping out these populations in large parts of the country," Peter Sibley from the Environment Agency said.
"Moving the native crayfish to this new habitat should mean they will be able to flourish without being threatened by the American crayfish or wiped out by disease. We hope this rescue bid pays off," he added.
Craig Stenson, from the Avon Wildlife Trust, added: "This is the first time a translocation has been attempted, rather than a re-introduction.
"Other populations of native crayfish are under threat - like those in the River Frome and Wellow Brook, south of Bath - but in the By and Broadmead Brooks the situation is perilous."