Hereditary ovalocytic red cells are characterized by a marked increase in membrane rigidity and resistance to invasion by malarial parasites. The underlying molecular defect in ovalocytes remained a mystery until Liu and colleagues (N. Engl. J. Med. 1990. 323:1530-38) made the surprising observation that the ovalocytic phenotype was linked to a structural polymorphism in band 3, the anion transporter. We have now defined the mutation in band 3 gene and established the biophysical sequelae of this mutation. This mutation involves the deletion of amino-acids 400-408 in the boundary between the cytoplasmic and the first transmembrane domains of band 3. The biophysical consequences of this mutation are a marked decrease in lateral mobility of band 3 and an increase in membrane rigidity. Based on these findings, we propose the following model for increased membrane rigidity. The mutation induces a conformational change in the cytoplasmic domain of band 3, leading to its entanglement in the skeletal protein network. This entanglement inhibits the normal unwinding and stretching of the spectrin tetramers necessary for membrane extension, leading to increased rigidity. These findings imply that the cytoplasmic domain of an integral membrane protein can have profound effects on membrane material behavior.