Finding the geometry of aquifers in an active volcano is important for evaluating the hazards associated with phreato-magmatic phenomena and incidentally to address the problem of water supply. A combination of electrical resistivity tomography (ERT), self-potential, CO2, and temperature measurements provides insights about the location and pattern of ground water flow at Stromboli volcano. The measurements were conducted along a NE-SW profile across the island from Scari to Ginostra, crossing the summit (Pizzo) area. ERT data (electrode spacing 20 m, depth of penetration of ~200 m) shows the shallow architecture through the distribution of the resistivities. The hydrothermal system is characterized by low values of the resistivity (2000 Ω m) except on the North-East flank of the volcano where a cold aquifer is detected at a depth of ~80 m (resistivity in the range 70--300 Ω m). CO2 and temperature measurements corroborate the delineation of the hydrothermal body in the summit part of the volcano while a negative self-potential anomaly underlines the position of the cold aquifer.