Infections with the microsporidian parasite Paranucleospora theridion are ubiquitous in farmed Atlantic salmon in western Norway Paranucleospora theridion infections show a clear seasonal pattern in parasite densities, being highest during autumn. Paranucleospora theridion densities are much higher in farmed salmon with low condition factors. Salmonid alphavirus infections and pancreas disease outbreaks are not governed by Paranucleospora theridion infections.