X-ray microanalysis using the analytical electron microscope (EMMA 4) was performed on resting frog sartorius muscles fixed by the pyroantimonate technique. Calcium was the main element of interest analyzed. Pretreatment was with ethyleneglycoltetraacetate to chelate calcium and with Tris chloride to replace sodium. Osmium-fixed controls were also analyzed. Both crystal-diffracting and energy-dispersive analyzers were employed, the former for accurate quantitation of calcium content and the latter for qualitative assessment of other elements present. Significant concentrations of calcium occurred in the dense parts of the nucleus, the triads, the thin filaments and the sarcolemma. The amount of calcium was greatly decreased by ethyleneglycoltetraacetate pretreatment and slightly increased by sodium removal. After osmium fixation alone, little calcium was detected with no localization.