Heat shock transcription factor I (HSF1) regulates heat shock proteins (HSPs), which assist in protein folding and inhibit protein denaturation following stress. HSFI was firstly cloned from orange-spotted grouper and exists as two isoforms, one with (osgHSF1a) and one without (osgHSF1b) exon 11. Heat exposure increased the expression of osgHSFlb while cold exposure increased that of osgHSFla. Both isoforms were mainly expressed in the brains, eyes, and fins. Expression of osgHSF1b was higher than osgHSFla during development. Poly I:C and LPS could also induce osgHSF1 isoforms expression differentially. Exposure to nervous necrosis virus (NNV) increased the level of both osgHSF1 isoforms at 12 h. GF-1 cells with overexpression of osgHSF1 isoforms enhanced viral loads within 24 h, whereas both pharmacological inhibition and RNA interference of HSF1 reduced virus infection. This study shows that osgHSF1 can support the early stage of virus infection and provides a new insight into the molecular regulation of osgHSF1 between the influence of temperatures and immunity. (C) 2016 Elsevier Ltd. All rights reserved.