The Environment Agency says that reports of "sewage" on beaches in north west Wales are due to algae forming off the coast.
When algae dies and breaks down, it forms a foam similar to sewage
The brown-coloured naturally-forming blooms smell strongly of rotten eggs and vegetables, leading to confusion, said the agency.
The reports were received along the Menai Straits on Anglesey and Tremadog Bay in Gwynedd.
Officials say the algae is harmless and non-toxic and they will continue to monitor its spread.
Warm weather and nutrients in the sea cause the algae, phaeocystis, to form, usually in June or July, although this type is called "May Bloom."
Sian Owens, from Environment Agency Wales said: "Every year we get reports of sewage slicks on inshore coastal waters, and of course we treat each one seriously and carry out tests.
"But almost always, despite its unpleasant appearance, the 'sewage' turns out to be this common May Bloom."