Warnings are being issued to stay away from blue-green algae that has been found at a loch in Twatt.
Shetland Islands Council said environmental health had been told by the Scottish Environment Protection Agency that the algae was in Collaster Loch.
Notices are to be posted in local shops and next to the loch warning that contact with algal scum should be avoided.
The council said adjoining landowners have been advised as have NHS Shetland.
It added at this stage there is no adverse effect on water supplies.
For further information contact environmental health on 01595 745250.