A glucose-6-phosphate isomerase deficiency is described in an Arab boy suffering from chronic hemolytic anemia. The patient was probably true homozygous for the defect. The residual enzyme activity in his red blood cells (RBC) was approximately 30% of normal. The most striking enzyme abnormality observed was an extreme heat lability: upon incubation at 45 C, greater than 90% of activity was lost within 15 min. Furthermore, an increased affinity for the substrate glucose-6-phosphate was shown. The lability of the enzyme was also shown to exist in vivo by separating the patient's RBC into four fractions of different cell age by centrifugation on a discontinuous density gradient. This in vivo lability of the enzyme is believed to be the main cause of the hemolytic diathesis. Remarkably, the residual activity of the enzyme in the RBC of obligate heterozygotes was comparable to that in the patient. However, their enzyme activity was only slightly more labile than that in normal RBC and consequently no signs of hemolysis were noticed.