A probe has been launched after more than 100 dead fish were discovered along a stretch of Shill Brook in Carterton.
Officers from the Environment Agency were yesterday carrying out tests on the water to find what caused the incident.
A member of the public called the agency after the fish - including brown trout - were spotted floating in the brook on Thursday.
Environment officer David Headon said: "This is a serious incident, which has affected a previously pristine habitat along the Shill Brook.
"We are investigating the possible cause of the fish kill and fisheries and environment officers are currently on site assessing the full impact of the incident."
Anyone who spots dead fish at other spots along the brook is asked to call the agency on 0800 80 70 60.
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