Natural Resources Wales (NRW) officers are investigating a slurry spill on a tributary of the Afon Taf, in Carmarthenshire, where more than 230 fish have been killed.
NRW officers identified the source of the slurry and the pollution has now stopped.
An ongoing assessment of the impact of the incident and fish kill, which included salmonids, bullheads, eels and lamprey, is being carried out over a 2km stretch of the Afon Hydron.
Andrea Winterton, South West Operations Manager for Natural Resources Wales, said:
“We identified the source of the pollution and made sure it had stopped and we will continue to monitor the river closely to ensure there are no ongoing environmental issues.
“Our investigations into how this incident happened are ongoing.
“If you see any pollution incidents you can report them to our incident hotline on 0800 807060.”