The discovery of more than 100 dead fish has sparked an investigation into the source of pollution in a brook.
The Environment Agency is running tests on the dead fish - including protected brown trout and bullhead - after the find in Shill Brook at Carterton.
Now anglers across the county are worried any pollution could be washed further downstream, killing more fish.
Dead fish were spotted floating on the surface of the brook on Thursday and when experts arrived they counted at least 100 dead fish.
Agency officers have now taken samples of the water to search for any pollution or contamination which has got into the brook and are running tests on the dead fish.
Steve Ferris, of the Abingdon and Oxford Angling Alliance, said: "The main thing is to find out what has caused this."
Environment Agency officer David Headon added: "We would ask anyone who sees dead fish to contact the Environment Agency on 0800 80 70 60."
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