Results of gravity and aeromagnetic measurements have been used for studying the subsurface shape of the Fellingsbro-Blixterboda granite massif, which was formed during the late stages of the Svecofennian orogeny, approximately 1800 Ma ago. The large negative gravity anomaly (ca. -120 gu) over the massif is interpreted to be caused by the relatively low density of the granite itself. The shape and size of the anomaly indicate that the exposed granite is thickest at its southern margin, reaching a depth of at least 15 km and possibly 20 km. To the north a considerable drop occurs in the gravity anomaly, suggesting a laccolithic form for the central and northern part of the granite massif. A main feature of the aeromagnetic anomalies over the Fellingsbro-Blixterboda granite is that they are conspicuously higher over the laccolithic part of the granite than over the intrusive, deep-going, stock-shaped granite, indicating a low magnetization of the latter part. Results of susceptibility determinations support this assumption. The origin and emplacement of the Fellingsbro-Blixterboda granite is apparently closely connected with the late orogeny of south central Sweden. The general shape of this granite massif is consistent with a mechanism of diapiric emplacement and a flow favoured by a zone of weakness in the Svecofennian basement.