A mycotic infection manifesting as abdominal distension with free serous fluid accumulation in the coelomic cavity is documented in farmed rainbow trout. Histological examination using PAS and silver staining revealed the presence of numerous fungal hyphae in the spleen and gastrointestinal wall. The isolated fungus was sterile and identified by using phylogenetic analysis based on four loci asNeopyrenochaeta submersa. This is the first time this fungus has been reported as pathogen.