This data set contains the 360 coefficients of the GSFC (12/66) spherical harmonics model of the Earth's main (core) magnetic field. The coefficients are for epoch 1960.0 and extend up to degree and order 10 in the constant, first derivative and second derivative terms. In addition to all available survey data from 1900 to 1964, the model is based on some Vanguard 3 and Alouette satellite data and on preliminary field observation by OGO 2 from October 29 to November 15, 1965. Use of this model beyond 1965 is not recommended. Geomagnetic field parameters can be generated with the ALLMAG programs (#PG-12A).