Authorities are investigating suspected pollution at Deenagh River in Killarney.
Inland Fisheries Ireland say there are no reports of dead fish.
It's one of three investigations launched by the IFI in the south-west.
Kerry County Council, the National Parks and Wildlife Service, and the Environmental Protection Agency are also involved in the suspected pollution probe at Deenagh River in Killarney.
Staff at Inland Fisheries Ireland are also investigating a fish kill at Curraghalicky Lake near Dunmanway in West Cork.
Several hundred juvenile bream have been recovered from the lake.
IFI staff are also investigating suspected pollution in the Ross River in Killavullen, North Cork following a report of heavy discolouration in the river.
No dead fish have been reported.
