Aluminum inhibited yeast glucose-6-phosphate dehydrogenase (EC 1.1.1.49) by a pseudo-first-order reaction. The inhibition was proportional to the incubation time and the concentration of aluminum. Double reciprocal plots gave a straight line with a kinact of 8.3 min-1 and indicated the presence of a binding step prior to inhibition. The kinetic study showed that 1 mol of aluminum was bound per mol of enzyme subunit.