Calsequestrin, the major Ca²⁺ storage protein of muscle, coordinately binds and releases 40–50 Ca²⁺ ions per molecule for each contraction-relaxation cycle by an uncertain mechanism. We have determined the structure of rabbit skeletal muscle calsequestrin. Three very negative thioredoxin-like domains surround a hydrophilic center. Each monomer makes two extensive dimerization contacts, both of which involve the approach of many negative groups. This structure suggests a mechanism by which calsequestrin may achieve high capacity Ca²⁺ binding. The suggested mechanism involves Ca²⁺-induced collapse of the three domains and polymerization of calsequestrin monomers arising from three factors: N-terminal arm exchange, helix–helix contacts and Ca²⁺ cross bridges. This proposed structure-based mechanism accounts for the observed coupling of high capacity Ca²⁺ binding with protein precipitation.