Opportunism is present in professional services and, as a result, organizations adopt mechanisms to constrain it. Despite the work on constraining opportunistic consultants, researchers have generally ignored the potential impact of national culture on the efficacy of constraint mechanisms. Using the theory of relationship constraints (TRC), this study examines the effectiveness of different constraint mechanisms for information systems (IS) consultants in the United States and China based on different levels of information asymmetry, tacit and explicit knowledge. While we found support in both cultures for the salience of these dimensions, we also found important distinctions for the effectiveness of different constraints between the cultures. Legal constraints are more effective in China while social constraints are more effective in the United States. While TRC is relevant in both cultures, national culture moderates the effectiveness of various constraint mechanisms and highlights the need for additional study.