Malignant hyperthermia (MHS) and environmental heat stroke (EHS) in humans present as similar life threatening crises triggered by volatile anesthetics and strenuous exercise and/or high temperature, respectively. Many families (70-80%) diagnosed with MH susceptibility (MHS), and a few with EHS, are linked to mutations in the gene that encodes the type 1 ryanodine receptor (RYR1) located in the sarcoplasmic reticulum (SR) of skeletal muscle. However, mutations in the RYR1 gene are not found in all MH families, suggesting that alternative genes remain to be identified. Here we investigated whether a MH/EHS-like phenotype results from deficiency in skeletal muscle calsequestrin (CASQ1), a SR Ca2+-binding protein that modulates RYR1 function. Exposure of CASQ1-null mice to halothane or heat stress triggers lethal episodes characterized by elevated core temperature, whole body contractures, and severe rhabdomyolysis. Both heat- and halothane-induced episodes are prevented by prior dantrolene administration, the standard antidote used to treat MH episodes in humans. In vitro studies indicate that CASQ1-null muscle exhibits increased contractile sensitivity to caffeine, temperature-dependent increases in resting Ca2+, and an increase in the magnitude of depolarization-induced Ca2+ release. These findings validate CASQ1 as a candidate gene for linkage analysis in MH/EHS families where mutations in RYR1 are excluded.