The endoplasmic reticulum (ER) serves as a cellular storehouse for Ca²⁺, and Ca²⁺ released from the ER plays a role in a host of critical signaling reactions, including exocytosis, contraction, metabolism, regulation of transcription, fertilization, and apoptosis. Given the central role played by the ER, our understanding of these signaling processes could be greatly enhanced by the ability to image [Ca²⁺]_ER directly in individual cells. We created a genetically encoded Ca²⁺ indicator by redesigning the binding interface of calmodulin and a calmodulin-binding peptide. The sensor has improved reaction kinetics and a K_d ideal for imaging Ca²⁺ in the ER and is no longer perturbed by large excesses of native calmodulin. Importantly, it provides a significant improvement over all previous methods for monitoring [Ca²⁺]_ER and has been used to directly show that, in MCF-7 breast cancer cells, the antiapoptotic protein B cell lymphoma 2 (Bcl-2) (i) lowers [Ca²⁺]_ER by increasing Ca²⁺ leakage under resting conditions and (ii) alters Ca²⁺ oscillations induced by ATP, and that acute inhibition of Bcl-2 by the green tea compound epigallocatechin gallate results in an increase in [Ca²⁺]_ER due to inhibition of Bcl-2-mediated Ca²⁺ leakage.