The calcium-sensitive bioluminescent protein aequorin has been microinjected into isolated amphibian twitch muscle fibers in order to detect the intracellular calcium transients associated with excitation-contraction coupling. While it is not yet possible to make quantitative estimates of the changes in calcium concentration involved, it is possible to detect substantial changes in intracellular calcium transients resulting from changes in temperature, stimulation frequency, fiberlength, and the osmotic strength of the bathing medium.