When wild-type zebrafish embryos are touched at 24 hours post-fertilization (hpf), they typically perform two rapid alternating coils of the tail. By contrast, accordion (acc) mutants fail to coil their tails normally but contract the bilateral trunk muscles simultaneously to shorten the trunk, resulting in a pronounced dorsal bend. Electrophysiological recordings from muscles showed that the output from the central nervous system is normal in mutants, suggesting a defect in muscles is responsible. In fact, relaxation in acc muscle is significantly slower than normal. In vivo imaging of muscle Ca2+ transients revealed that cytosolic Ca2+ decay was significantly slower in acc muscle. Thus, it appears that the mutant behavior is caused by a muscle relaxation defect due to the impairment of Ca2+ re-uptake. Indeed, acc mutants carry a mutation in atp2a1 gene that encodes the sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1), a Ca2+ pump found in the muscle sarcoplasmic reticulum (SR) that is responsible for pumping Ca2+ from the cytosol back to the SR. As SERCA1 mutations in humans lead to Brody disease, an exercise-induced muscle relaxation disorder, zebrafish accordion mutants could be a useful animal model for this condition.