The Santiaguito volcanic dome insouthwestern Guatemala is a multiple extrusive dome that has shown constant volcanic activity since its birth in 1922. Fourteen extrusive units are mapped. Five of these are volcanic domes; the remaining nine are lava flows which generally cling to the sides of the domes. The volume of material extruded at Santiaguito since 1922 (0.7 km3) is a small fraction of the volume of pyroclastic debris from the 1902 eruption of Santa Maria (5.5 km3), Santiaguito's parent composite cone. Extrusion of the dome began in the center of the explosion crater created by Santa Maria's 1902 activity. This crater was volumetrically much smaller (0.5 km3) than the amount of material erupted during the 1902 event, and local slumping near the crater has occurred and is continuing along a series of east-trending faults. The general westward growth of the dome complex and many of the structural featureson Santiaguito are controlled by these near-vertical faults. The domes are studded with Pelean spines. Twenty-five new chemical analyses are presented, showing Santiaguito's eruptive products to be soda-rich dacite of the calc-alkaline suite. The dome lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa Maria and the older volcanic rocks in the area. Trace element analyses of the lavas, along with major element data suggest a differentiation by fractional crystallization under constant or increasing PO2. Sr isotope determinations could not detect contamination of the lavas by radiogenic crustal material; the Sr87/Sr86 ratio in Santiaguito rock averages .7043. The Santiaguito lavas are tridymite-bearing hypersthene dacites, characterized by strongly zoned plagioclase phenocrysts and oxyhornblende. Compositional and petrographic comparisons are made with other modern volcanic domes and other Central American lavas.