Routine monitoring by Argyll and Bute Council has found raised levels of naturally occurring algal toxins in two areas.
Loch Melfort and Seil Sound have been found to have the raised levels.
It means shellfish have toxins levels above the legally permitted levels for harvest and members of the public are asked not to harvest them.
The naturally occurring toxins accumulate in molluscs such as mussels, oysters, cockles and razor clams. When levels breach statutory limits, harvesting of affected species is prohibited.
Elsewhere, raised levels of Escheria coli (E.coli) bacteria have been identified in Oitor Mhor Bay.
It follows discovery of raised levels at Kerrera West, Loch Craignish, Kilfinichen Bay and Castle Stalker.
All raised levels were discovered due to monitoring work by Argyll and Bute Council’s Environmental Health team, carried out as part of its work to maintain the high quality of Argyll shellfish.
In addition, a potential risk of raised levels of E.coli has been found at Dunstaffange, Kerrera East, Loch na Cillie and Loch Riddon.
Eating shellfish such as cockles, mussels, oysters and razor fish from affected areas can pose a risk to human health.
Notices to warn the public and casual gatherers have been posted along the shore. Commercial shellfish harvesters in these areas have been contacted by the Council and steps taken to postpone harvesting until bacterial levels subside.
People are advised to avoid eating shellfish from these areas until further notice as a precaution.
When levels subside, the warning notices will be removed.
For further information, contact the Environmental Health Team on 01631 567849. Food Standard Scotland can also be contacted on 01224 285100 or at the organisation’s website.