Environment Agency Wales is appealing to the public for information after more than 100 fish were found dead at Nant Cwm Parc near Commercial Street, Senghenydd.
This follows information from a member of the public who contacted the Agency to report a number of dead fish and cloudy water in Cwm Parc stream.
Environment Agency Wales officers are now back on site after initial investigations on Wednesday.
It is looking into the source of possible pollution and the impact it may have had on the water. The agency believes that any pollution has now stopped and the water is clear again.
Anyone with further information about this incident should contact Environment Agency Wales on 0800 80 70 60. Calls will be treated in the strictest confidence.
The Agency is reminding the public that if they see dead fish in rivers or streams they should not attempt to remove them and should not be consumed in any instance.