The initial rates of isomerization between glucose 6-phosphate and fructose 6-phosphate catalyzed by Bacillus stearothermophilus phosphoglucose isomerase (PGI) were measured in both the forward and reverse reactions. Although B. stearothermophilus PGI is a tetrameric enzyme, the reaction rate vs substrate concentration curves for both reactions exhibited Michaelis-Menten kinetic behavior. This was confirmed by the Hill plot which gave the Hill coefficient of 1.0 for both reactions. Based on the above experimental results and another experimental result that the number of substrate or product binding sites on the PGI molecule was 4, we propose a reaction scheme which is able to explain Michaelis-Menten kinetic behavior of this oligomeric enzyme, and determine the kinetic parameters.