A scalar magnetic anomaly map for South America and adjacent marine areas was prepared from MAGSAT data. Preparation of the map posed problems, notably in separating external field and crustal anomalies, and in reducing data to a common altitude. External fields are manifested in a long-wavelength ring current effect, a medium-wavelength equatorial electrojet, and short-wavelength noise. The ring current is corrected through use of a standard ring current equation, augmented by further wavelength filtering. Because the electrojet is confined primarily to dusk profiles, its effect is minimized by drawing the data set from dawn profiles only. The noise is reduced by selecting profiles from 'quiet' periods (K < or = 2 (super +) ). The optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength. When wavelengths longer than this are rejected, resultant profiles from redundant satellite orbits correlate well, with differences due primarily to satellite altitude. These differences are eliminated by normalizing the data to a constant altitude by the equivalent point-source inversion technique. The resultant map differs significantly from the original 2 degree averaged version, most notably in low geomagnetic latitudes. A version of this map reduced to the pole shows correlations with several regional South American and Caribbean tectonic features.