Pancreatic disease (PD) is suspected at two of Mowi's salmon farms at locations in Nordland, northern Norway.
The sites are located in a protection zone established after PD was suspected nearby in September.
Both localities are subject to restrictions such as bans on moving fish to prevent possible spread of infection.
Mowi notified the Norwegian Food Safety Authority of its PD suspicions Nov. 16, based on positive results from tests it had carried out itself.
Over the next few days, the Norwegian Food Safety Authority will take verifying samples, which will be sent to the Veterinary Institute for analysis.