The distribution and migration of radon and helium soil-gas concentrations in the Phlegraean Fields, Italy, are controlled by the tectonic features of the area. Radon is supplied from surficial sources and helium has both surficial and deep origins. There is no direct correlation between the two noble gases on a point-to-point basis but the areal distribution of both gases is similar, suggesting that the distribution is controlled primarily by fractures andd movement of geothermal fluids.