Juvenile salmon migrating from freshwater to the marine environment confront a marked change in environmental osmolality. Using differential display of mRNA expression, we cloned a 1.9 kb cDNA upregulated in isolated tissues of salmon exposed to the hyperosmotic stress associated with transition to the dehydrating marine environment. The cDNA codes for a 21 kDa protein (Shop21) with 98% identity to Rbx1, an E3 ubiquitin ligase; the protein also contains a novel 81 amino acid domain at the NH terminus not found in Rbx1. Moderate hyperosmotic stress (24h at 550 mOsm/kg) increased Shop21 transcript 10-fold in branchial lamellae whereas no upregulation was observed under more severe stress (800 mOsm/kg). Expression of the gene also was observed in heart and kidney. Replacement of NaCl with mannitol, but not glycerol, also elicited an increase in Shop21 mRNA. Inhibition of the MAPK and MEK signal transduction pathways failed to blunt the Shop21 response during hyperosmotic stress. Of note, Shop21 mRNA also accumulated during thermal stress, but to a lesser extent than hsp70 mRNA. The potential importance of Shop21 to the living animal is suggested by marked upregulation of the gene in salmon following transfer to seawater. The results of these investigations suggest that Shop21 may have a role in targeting selected proteins (e.g., in freshwater ionocytes) non-essential for adaptation to seawater, for removal via the proteasome pathway.