The magnetic aspect angle sensitivity of 3-m plasma density irregularities in the F-region ionosphere over Jicamarca has been measured during the passage of a radar plume in an equatorial spread-F event. The measurement technique utilizes radar interferometry with a number of antenna baselines with different lengths and orientations. Several corrections are applied to the data to reduce experimental biases. The RMS aspect angle half widths (the square root of the angular variance, the standard deviation) were found to be 0.01±0. 005° in a bottomside layer and near the top of the plume and 0.02±0.005° in the central channel of the plume near F-peak altitudes. In the frequency domain, the magnetic aspect width was narrowest at small Doppler shifts and broader in the wings of the spectra when wings existed. These findings appear to be reasonably consistent with theoretical predictions, although questions remain. Copyright 2011 by the American Geophysical Union.