Data from the Magsat spacecraft for November 1979 through April 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth's main magnetic field and its secular varition at epoch 1980.0. The model is called GSFC(12/83). Constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10. The first-degree external terms and corresponding induced internal terms are given as a function of Dst. Preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current. The final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods: (1) November-December 1979, (2) January-February 1980, and (3) March-April 1980. A correction for the effects of the equatorial electrojet was applied to the dusk data utilized. The solution included calculation of fixed biases, or anomalies, for the observatory data. Although similar in many respects, GSFC(12/83) differs from International Geomagnetic Reference Field 1980 by 3.6 nT in the g10 term and shows a slighlty negative B in the northern polar region as well as other differences in secular variation pattern.