Cross-cultural trust and cooperation are important concerns for international markets, political cooperation, and cultural exchange. Until recently, this problem was difficult to study under controlled conditions due to the inability to conduct experiments involving interaction between participants located in physically distant locations. We report results of an experiment using a Web-based "virtual lab" to study trust and trustworthiness between Japanese and Americans in real-time interaction. Participants played a variation of the Trust Game in two different experimental conditions: a "flags-on" condition in which everyone's nationality was publicly identified during the session, and a "flags-off" condition in which participants did not know who was Japanese or American. The findings most strongly support Yamagishi's structural theory of trust, which predicts that Japanese will form more durable exchange relations compared to Americans. We found less support for explanations that focus on cultural differences in trust and trustworthiness, and for cognitive explanations that point to the effects of a shared social identity between participants and partners with common nationality. (Contains 20 footnotes, 6 tables, and 1 figure.)