Background measurements of gamma radiation were made on the Jordán (Upper Permian ?), Guatapurí (Triassic ?), Girón (Triassic-Jurassic), Ranchogrande (Triassic-Jurassic ?) and Morrocoyal (Lower Jurassic) Formations in northeastern Colombia. The lutites of the Girón Formation produce the highest values (impulses y per minute). In the region of Bucaramanga (Dept. of Santander), the redbeds of the Jordán and Girón Formations give quite different radiometric values. The usefulness of radiomatric measurements as a means of differentiating stratigraphic units and facies changes is discussed.