Elevated intracellular Ca²⁺ triggers inactivation of L-type calcium channels, providing negative Ca²⁺ feedback in many cells. Ca²⁺ binding to the main α_1C channel subunit has been widely proposed to initiate such Ca²⁺-dependent inactivation. Here, we find that overexpression of mutant, Ca²⁺-insensitive calmodulin (CaM) ablates Ca²⁺-dependent inactivation in a "dominant-negative" manner. This result demonstrates that CaM is the actual Ca²⁺ sensor for inactivation and suggests that CaM is constitutively tethered to the channel complex. Inactivation is likely to occur via Ca²⁺-dependent interaction of tethered CaM with an IQ-like motif on the carboxyl tail of α_1C. CaM also binds to analogous IQ regions of N-, P/Q-, and R-type calcium channels, suggesting that CaM-mediated effects may be widespread in the calcium channel family.