People in Oban have complained to Scottish Water after finding midge larvae in their drinking supply.
Customers claim about two or three of the tiny organisms, up to 6mm long, are being found in every glass of water.
The problem occurred after a large amount of the larvae clogged a filter at the local reservoir.
Scottish Water said the creatures, known as bloodworms, were harmless and it had added a backwash of chlorine to the supply to stop them from breeding.
Up to 30 customers in parts of Oban have contacted Scottish Water in recent days querying the quality of their drinking water.
Resident Scott Hetherington, 35, said the worms were difficult to see.
"In a glass of water they are very, very difficult to spot," he said.
"In the quantities they're coming through, what they've started to do is cluster together into balls and it's very, very easy to see balls of 20 or 30 of these.
"It is still a serious problem and Scottish Water are not informing their customers at all."
Some residents have been offered bottle water until the problem improves.
Scottish Water said it had taken samples from the homes of all those who have complained.
Regional manager Steven Fraser said: "The particles in the water are the larvae of midges.
"Normally these are filtered out at the treatment works but an abnormal amount appears to have clogged one of our filters, allowing some to enter the supply."
Mr Fraser apologised for any stress it had caused customers but said it was an "aesthetic issue" and there was no risk to health.
Scottish Water said it has increased the frequency of filter cleaning and they are confident the problem won't happen again.