A 2.2-kb cDNA clone encoding the endoplasmic reticulum (ER) resident protein, calnexin (CLX), was isolated from the frog Rana rugosa liver cDNA library and sequenced. The sequence encoded 622 amino acids and was 77% similar to mammalian CLXs and 56% to mouse CLX-t. In the phylogenetic tree, mouse CLX-t was clearly diverged from other CLXs including frog. The amino acid sequence of CLX showed regions similar to those in frog calreticulin (CLT), although CLX, but not CLT, contained a histidine-rich region at the NH2-terminus. CLX gene expression was observed only in the liver among various tissues examined. Additionally, its expression was strong in the liver in 2-month post-metamorphosis frogs, but very weak in adults. The results suggest that the CLX gene is expressed in a tissue- and stage-dependent manner in the frog, R. rugosa, and that CLX is widely distributed in eukaryotic organisms.