Inland Fisheries Ireland is investigating a significant fish kill incident on River Erkina, near Rathdowney in Co Laois.
Environmental and fisheries officers from the South-Eastern River Basin District were alerted by a member of the public earlier this week.
Inland Fisheries Ireland estimates that there could be in excess of 2,000 trout killed in the incident, in addition to other species, such as salmon parr, lamprey and crayfish.
The State agency is responsible for the protection and conservation of freshwater fish and habitats.
It said that fish species in the Erkina River are currently very vulnerable due to higher water temperatures and lower water levels at this time of year.
The agency said that if there is less oxygen in the water, fish find it extremely difficult to survive.
It said: "Investigations into this fish kill incident are ongoing and Inland Fisheries Ireland is not in a position to comment on the cause at this stage."
It has encouraged members of the public to report fish kills or water pollution on its 24-hour hotline 0818 34 74 24.