People are being warned to keep away from the polluted River Kennet between Hungerford and Marlborough after a potentially harmful pesticide spill.
The pesticide is believed to have leaked into the river near sewage treatment works at Marlborough affecting eight miles of water.
The Environment Agency is investigating the source of the pollution, believed to be an agricultural pesticide.
Spokesman John Sutton urged people to avoid skin contact with the water.
Mr Sutton said: "It's a precautionary approach but it does seem sensible at this point, people should also not eat fish caught from that section of the river."
"Our focus is still very much trying to track down the source of this serious pollution.
"It looks like something that has been tipped or washed into that drainage system."
The alert was raised on Monday by volunteer river fly monitors from Action for the River Kennet (ARK).
ARK said its monthly check of the water showed freshwater shrimp, mayfly and caddis fly in about 15km (nine miles) of the river appeared to have been wiped out.
It believes there has been no impact on fish in the river.
The chalk stream is a Site of Scientific Interest.