Jupiter is a copious source of radiowaves having a wavelength of 3–75 cm. The radiation is 20% or so linearly polarized perpendicular to the planet rotation axis, does not vary much in intensity over the entire range of observed frequencies, and changes slowly in intensity over a time interval of the order of a few years. Chang and Davis (1962) have shown that the source is a belt of very energetic electrons trapped in the dipole field of the planet radiating by synchrotron emission. Spatially resolved observations at different frequencies have suggested to us a method by which the electron number density and energy can be determined everywhere in the equatorial plane.