Distribution of infectious hematopoietic necrosis (IHN) was studied in five major provinces producing rainbow trout(Oncorhynchus mykiss)fry in Iran by indirect fluorescence antibody IFATand nested-RT-PCR techniques. Also the effect of time duration was examined on some positive samples after 8 months post sampling. Samples of kidney, liver and spleen of rainbow trout fries collected from 27 trout farms were processed and examinated. Fourteen trout hatcheris located in all provinces were identified to be contaminated with IHNV tested by both techniques. The obtained results, also showed that nested-RT-PCR is more sensitive than IFATtest particularly if samples are stored for a longer time.