Phosphoglucoisomerase from cytosol of immature wheat endosperm was purified 650-fold by ammonium sulphate fractionation, isopropyl alcohol precipitation, DEAE-cellulose chromatography and gel filtration through Sepharose CL-6B. The enzyme, with a molecular weight of about 130,000, exhibited maximum activity at pH 8.1. It showed typical hyperbolic kinetics with both fructose 6-P and glucose 6-P withKm of 0.18 mM and 0.44mM respectively. On either side of the optimum pH, the enzyme had lower affinity for the substrates. Using glucose 6-P as the substrate, the equilibrium was reached at 27% fructose 6-P and 73% glucose 6-P with an equilibrium constant of 2.7. The ΔF calculated from the apparent equilibrium constant was +597 cal mol-1. The activation energy calculated from the Arrhenius plot was 5500 cal mol-1. The enzyme was completely inhibited by ribose 5-P, ribulose 5-P and 6-phosphogluconate, withKi values of 0.17, 0.25 and 0.14 mM respectively. The probable role of the enzyme in starch biosynthesis is discussed.