A three-component magnetometer was carried on Explorer 12, launched on August 16, 1961. The boundary of the geomagnetic field on the sunlit side of the earth was observed. During magnetically quiet periods the boundary, near the earth-sun line, was at approximately 10 earth radii distance. During the period of observation a field of magnitude 30 to 40 gammas existed outside the boundary; it was usually directed downward, or southward, from the ecliptic plane. The boundary was stable, and magnetic fluctuations inside the geomagnetic field were relatively small although often large outside.