Bilateral gamma process is widely used in risk management and asset pricing. However the behavioral implications of this process remain unknown. This paper investigates this problem for the first time within the framework of Tauchen and Pitts (1983). With the assumption that there are two types of traders in the market, the optimistic and the pessimistic, we find the bilateral gamma process can be derived from Walrasian equilibrium. This finding establishes the microstructure foundations for the bilateral gamma process.