We have generated a line of mutant mouse that lacks Klotho, a protein that structurally resembles Klotho. The synthesis and excretion of bile acids were found to be dramatically elevated in these mutants, and the expression of 2 key bile acid synthase genes, cholesterol 7␣-hydroxylase (Cyp7a1) and sterol 12␣-hydroxylase (Cyp8b1), was strongly upregulated. Nuclear receptor pathways and the enterohepatic circulation, which regulates bile acid synthesis, seemed to be largely intact; however, bile acid-dependent induction of the small heterodimer partner (SHP) NR0B2, a common negative regulator of Cyp7a1 and Cyp8b1, was significantly attenuated. The expression of Cyp7a1 and Cyp8b1 is known to be repressed by dietary bile acids via both SHP-dependent and -independent regulations. Interestingly, the suppression of Cyp7a1 expression by dietary bile acids was impaired, whereas that of Cyp8b1 expression was not substantially altered in klotho؊/؊ mice. Therefore, Klotho may stand as a novel contributor to Cyp7a1-selective regulation. Additionally, Klotho-knockout mice exhibit resistance to gallstone formation, which suggests the potential future clinical relevance of the Klotho system.