Fluctuations in the neutron flux caused by steam bubbles were analyzed to infer the average void fraction in the four fuel bundles that surround an in-core detector string in a boiling water reactor. The velocity of steam bubbles was inferred from the phase lag between axially displaced in-core fission detectors. This velocity, together with the measured power distribution and mass flow rate, was used to obtain the void fraction as a function of axial position. The results are in agreement with the predictions based on the Zuber et al. model, except near the top of the fuel channel.