Auditors need to be professionally skeptical when they are conducting an audit. Skeptical behavior is especially needed when there are indications of fraud in the audited entity. Without skepticism, fraud is likely to be overlooked, because fraud is usually concealed by the perpetrators. This experimental study aimed to investigate the influence of skeptical attitude, tone at the top, and knowledge of fraud on auditors' professional skeptical behavior. The study participants were junior auditors, senior auditors, and auditor supervisors. The analysis led to three key findings in this study. First, skeptical attitude, tone at the top, and the auditor's knowledge of fraud influenced the skeptical behavior of professional auditors. Second, a high tone at the top of the partner's style played an important role in maintaining and improving auditors' professional skeptical behavior, particularly for auditors who had a weak skeptical attitude. This result supported the theory of the attitudes-behavior relationship described by Fazio (1990) in the MODE model. Third, auditors' knowledge about fraud improved their professional skeptical behavior. However, knowledge of fraud did not influence auditors' professional behavior if those auditors had weak skeptical attitudes and were supervised by a supervisor with a low tone at the top style. Keywords: Tone at The Top, The MODE Model, Professional Skepticism, Knowledge of