A single MIPS gene (Isyna1/Ino1) exists in eel and tilapia genomes with a single MIPS transcript identified in all eel tissues, although two MIPS spliced variants (termed MIPS(s) and MIPS(l)) are found in all tilapia tissues. The larger tilapia transcript (MIPS(l)) results from the inclusion of the 87-nucleotide intron between exons 5 and 6 in the genomic sequence. In most tilapia tissues the MIPS(s) transcript exhibits much higher abundance (generally >10-fold) with the exception of white skeletal muscle and oocytes where the MIPS(l) transcript predominates. SW-acclimation resulted in large (6- to 32-fold) increases in mRNA expression for both MIPS(s) and MIPS(l) in all tilapia tissues tested whereas in the eel changes in expression were limited to a more modest 2.5-fold increase and only in the kidney. Western blots identified a number of species- and tissue-specific immunoreactive MIPS proteins ranging from 40 to 67 kDa molecular weight. SW-acclimation failed to affect the abundance of any immunoreactive protein in any tissue tested from the eel. However, a major 67 kDa immunoreactive protein (presumed to be MIPS) found in tilapia tissues exhibited 11- and 54-fold increases in expression in gill and fin samples from SW-acclimated fish. Immunohistochemical investigations revealed specific immunoreactivity in the gill, fin, skin and intestine taken from only SW-acclimated tilapia. Immunofluorescence indicated that MIPS was expressed within gill chondrocytes and epithelial cells of the primary filaments, basal epithelial cell layers of the skin and fin, the cytosol of columnar intestinal epithelial and mucous cells as well as unknown entero-endocrine-like cells.