Diffusion perpendicular to the magnetic field is investigated in a potassium plasma of non-uniform density in which the "universal" instability is excited. A qualitative picture of such diffusion is obtained by studying the relationship between the radial distribution of plasma density and the magnetic field, and by comparing the experimental densities with calculated ones on the assumption of classical diffusion. For densities of 109—1010 cm−3, the radial distribution and the peak density do not depend on the magnetic field. At higher densities, however, the half-width of the distribution becomes smaller and the peak diminishes as the field increases. At densities of 7 × 1010—5 × 1011 cm−3 the curves for the radial distribution agree well with the calculations.Direct measurements of the diffusion coefficient come from measurements of the plasma flux perpendicular to the magnetic field. In the density range 109—1010 cm−3, the diffusion coefficient does not depend on magnetic field or on density. At higher densities it does depend on the magnetic field H, and at densities greater than 2 × 1011 cm−3 a 1/H2 relationship is observed; the diffusion coefficient agrees with the classical coefficient in absolute value. The spread in the diffusion coefficients obtained from a single series of experiments, representing the statistical accuracy of the measurements and the stability of the diffusion coefficient, is ∼40%. The average value of the diffusion coefficient for low densities is (2.3 ± 0.9) × 102 cm2/s, greater than the classical coefficient by 1 to 2 orders of magnitude. The diffusion coefficient and the oscillation amplitude are correlated. As density is varied they remain constant until a certain critical density is reached, after which they begin to decline simultaneously. Classical diffusion is observed in the region where instability is control. Ion-ion diffusion and wall diffusion play no part in the experiments. The conclusion is drawn that the diffusion coefficient measured in the density range 109—1010 cm−3 is the result of the universal instability in a collisionless plasma.