BK potassium channels are implicated in a variety of physiological processes and they have been characterized in many species from Drosophila to humans although detailed studies about their properties and functions in the osmoregulation of teleosts are still lacking. In this study, we have cloned 911 bp corresponding to a partial Salmo salar BK channel coding sequence with high level of homology to BK channel sequences from other teleosts. Also, we have followed the BK gene expression pattern in gills of Atlantic salmon during brackish water (BW) acclimation. Interestingly, transfer from freshwater to BW induced an upregulation in mRNA BK channel expression in gills 7 days after transfer to BW, followed by a decrease in the expression. These results are the first describing mRNA changes of BK channel in gills of Atlantic salmon during BW acclimation, and they could suggest a possible role for BK channel during this adaptive response. Additionally, our results could position BK potassium channels expressed in the gills of Salmo salar as new candidates to be a marker of smoltification process, which may complement, and possibly outperform NKA activity measurement, the current gold standard of the Chilean industry.