The genes encoding the type I and type II interferons (IFNs) have previously been identified in rainbow trout and their proteins partially characterized. These previous studies reported a single type II IFN (rtIFN-γ) and three rainbow trout type I IFN genes that are classified into either group I (rtIFN1, rtIFN2) or group II (rtIFN3). In this present study, we report the identification of a novel IFN-γ gene (rtIFN-γ2) and a novel type I group II IFN (rtIFN4) in homozygous rainbow trout and predict that additional IFN genes or pseudogenes exist in the rainbow trout genome. Additionally, we provide evidence that short and long forms of rtIFN1 are actively and differentially transcribed in homozygous trout, and likely arose due to alternate splicing of the first exon. Quantitative reverse transcriptase PCR (qRT-PCR) assays were developed to systematically profile all of the rainbow trout IFN transcripts, with high specificity at an individual gene level, in naïve fish and after stimulation with virus or viral-related molecules. Cloned PCR products were used to ensure the specificity of the qRT-PCR assays and as absolute standards to assess transcript abundance of each gene. All IFN genes were modulated in response to Infectious hematopoietic necrosis virus (IHNV), a DNA vaccine based on the IHNV glycoprotein, and poly I:C. The most inducible of the type I IFN genes, by all stimuli tested, were rtIFN3 and the short transcript form of rtIFN1. Gene expression of rtIFN-γ1 and rtIFN-γ2 was highly up-regulated by IHNV infection and DNA vaccination but rtIFN-γ2 was induced to a greater magnitude. The specificity of the qRT-PCR assays reported here will be useful for future studies aimed at identifying which cells produce IFNs at early time points after infection. © 2008 Elsevier Ltd.