We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs.The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed.Carbon isotopic composition shows an unusually wide range-from -42.9 to + 13.5% O -in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18 O from + 3.4 to + 7.60% 0 in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative δ 13 C value was derived from biochemical oxidation of methane with a negative δ 13 C value. On the other hand, carbon with positive δ 13 C value was probably formed during methane production in an anoxic condition.