Hundreds of fish have been killed by pollution in the River Gwili in Carmarthenshire.
Natural Resources Wales (NRW) is investigating the incident after receiving reports of pollution of the tributary near Llanpumsaint.
About 200 trout, 40 lamprey and hundreds of bullheads have been killed.
NRW said officers identified the pollution as slurry from a nearby farm and have stopped any further pollution.
Kimberley Redman, natural resource management team leader for NRW, said the pollution has had a "significant impact" on fish in the river.
She added: "Following quick action from our officers to identify the source and stop the pollution, it's unlikely that we'll see further impact."