Novel rod-shaped calcite crystals are formed by precipitation from cetyltrimethylammonium bromide ( CTAB)/1-pentanol/cyclohexane microemulsions containing calcium chloride and ammonium carbonate. The calcium carbonate initially precipitates as hexagon-shaped vaterite crystals. The vaterite crystals transform to unusual rod-shaped calcite crystals over several days. The rod-shaped calcite crystals are prismatic, with the longest crystal axis displaying ( 110) crystal faces. A possible mechanism of crystal growth is discussed. The elongated shape of the crystals facilitates the assembly into hierarchical structures and can allow the crystals to be used as templates for fabricating advanced materials.