The Environment Agency has traced the source of a diesel spill to a Co Antrim river.
Anglers say a considerable volume of diesel found its way into the Six Mile Water at Ballyclare last night.
It's the second time in recent weeks that the river has been affected by fuel oil.
The Northern Ireland Environment Agency said it was called to the scene on Monday night after being notified by the public.
Its officials found the red diesel discharging to the river from a culvert.
Booms were put in place to contain the pollution and the source was traced to a business premises.
Officials there found "a significant amount of oil" which was contained and removed.
A spokesperson for Asda said: "We are supporting authorities with the clean-up operation that is taking place next to our store in Ballyclare."
The Fire and Rescue Service confirmed that two tenders had been sent to the scene last night after a report that diesel had leaked from a tank and into storm drains.
The matter will now be the subject of an investigation and possible prosecution.
It's not believed any fish were killed by the pollution, but it does pose a threat to other aquatic life.
The Six Mile Water is a wild trout river and supports important populations of otter and kingfisher.
Traces of diesel could still be seen in the river many miles downstream at Templepatrick this morning.
An investigation is underway to establish if yesterday's incident and a similar one in January are linked.
Jim Gregg of the Six Mile Water Trust visited the scene last night. He said when he arrived the culvert was "pure red" and contained a significant amount of diesel.
He said he was "furious" that the river should be treated in such a way.
Antrim and Newtownabbey Borough Council has warned people not to let their pets into the river which runs through the town's parks.
The Six Mile Water rises near Larne and flows past the towns of Ballyclare, Ballynure, Templepatrick and Antrim to Lough Neagh.