Radon is a recently recognized hazard of geological origin, regarded as the second most important cause of lung cancer. In this study, we investigate the relationship between geologic conditions and radon concentrations in groundwaters using geostatistical methods. The study area, situated in the western part of Ardennes, Belgium, consists mainly of phyllites and quartz-phyllites from the Lower Devonian period. These geological environments are considered to be a source for radon concentration from a previous study. More than 330 water samples from different springs have been taken by our laboratory in the study area (>400 km2) from 1984 to 1987. Results from this survey led to the discovery of uranium mineralization and a risk zone with many dwellings showing high radon concentrations. Radon concentrations of all samples are adequately represented by a lognomal distribution. Experimental variograms of the log-transformed values are calculated for four directions in a two-dimensional space. The variograms are fitted using the weighted least-squares method. A model for the whole area is composed of two spherical models, each of which is of geometric anisotropy. Finally, logarithmic kriging, a geostatistical technique, is conducted. The results of kriging mapping show that higher radon concentrations are evidently situated in the quartzphyllites of Braux, a formation of Lower Gedinnian fractured in particular in the saddle of an anticline with minor folds. So the Lower Gedinnian quartz-phyllites have the highest level of radon concentrations in the area. The results of this work may assist in defining and predicting areas with high potential for uranium exploration and indoor radon risk zones.