Increases in branchial Na+/K+ ATPase activity during seawater adaptation of euryhaline fish species, have been well documented. During the parr-smelt transformation of salmonids this activity increases two to five fold and is used as an indicator of the transformation. In order to improve the understanding of differences in enzyme activity found between Atlantic salmon Salmo salar parr and smelt fish, we investigated the gene expression of the Na+/K+ ATPase alpha-subunit(s) in gill tissue. Gill mRNAs were analyzed and quantified at distinct time points using Northern and Dot blot techniques. We amplified by PCR, a conserved region of the cDNA encoding the Na+/K+ ATPase alpha-subunit of the rainbow trout Oncorhynchus mykiss. The PCR products (670 bp) were cloned and all independent clones showed a sequence corresponding to the alpha subunit of the Na+/K+ ATPase. The fragments obtained appeared as a heterogenous population of three sequences showing, when compared between each other, 86 to 93% identity. This suggests that different allelic forms of the alpha-subunit are expressed in gill tissue. Hybridization studies performed with these PCR probes revealed two mRNA species, a major 3.7 kb transcript and a minor transcript of 1.8 kb. Enhanced 3.7 kb transcript levels are concurrent with elevated enzyme activity in smelts during the March and April parr-smolt transformation of Atlantic salmon. Interestingly, our study disclosed that smelt fish only displayed a two-fold increase in transcript levels when compared to parr whereas enzyme activity showed a 4 to 5 fold increase. This suggests that the increase in the 3.7 kb mRNA content of gill tissue is probably not the only mediator leading to the rise in enzyme activity during parr-smelt transformation.