The magnetic field and the transmembrane action potential of a single nerve axon were measured simultaneously. The volume conductor model was used to calculate the magnetic field from the measured action potential, allowing comparison of the model predictions with the experimental data. After analyzing the experiment for all systematic errors, we conclude that the shape of the magnetic field can be accurately predicted from the transmembrane potential and, more importantly, the shape of the transmembrane potential can be calculated from the magnetic field. The data are used to determine r_i, the internal resistance per unit length of the axon, to be 19.3 ± 1.9 k Ω mm^-1, implying a value for the internal conductivity of 1.44 ± 0.33 Ω^-1 m^-1. Magnetic measurements are compared with standard bioelectric techniques for studying nerve axons. © 1985 Biophysical Society