Summary: This paper shows that if rationality is not common knowledge, the no-trade theorem of Milgrom and Stokey fails to hold. We adopt Monderer and Samet’s notion of common \(p\)-belief and show that when traders entertain doubts about the rationality of other traders, arbitrarily large volumes of trade as well as rationality may be common \(p\)-belief for a large \(p\). Furthermore, rationality and trade may simultaneously be known to arbitrary large (but finite) degree.