Hyphal growth of Candida albicans was observed at neutral condition, whereas the yeast growth was increased below pH 5. Eight out of 12 strains of C. albicans grow in hyphal form at pH 7, and hyphal formation was inhibited in all strains at pH 4. The addition of GMP, an alternative oxidase (AOX) activator, induced the hyphal growth at pH 4. Although C. albicans grew in hyphal form in pH 7, SHAM, an AOX inhibitor, enhanced the yeast proliferation at this pH. The relative expression level of RAS1 mRNA was higher at pH 7 than at pH 4, indicating that the hyphal formation signal was defective under acidic conditions. Based on these findings, we concluded that AOX-RAS1 signal transduction is the main pathway of hyphal formation of C. albicans, and that the signal was controlled by pH condition.