While conducting a monitoring program in arid southwestern Madagascar we observed a large aggregation of ca. 100 radiated tortoises (Astrochelys radiata) along ca. 80 m of dry creek bottom at the onset of the wet season. Tortoises had apparently anticipated rainfall and as precipitation ensued, immediately began drinking from small depressions in the bedrock. We discuss potential mechanisms and consequences of the behavior and compare it with similar observations in desert tortoises (Gopherus agassizii).