The paper describes the first measurements of beta and diamagnetism in beam heated current-less plasmas of Heliotron E. These measurements were performed with a diamagnetic coil (1984–1985). The inferred volume average beta shows good agreement with the kinetic beta determined from the Thomson scattering electron temperature and density and the ion temperature profiles. A volume average beta of ⟨βτ⟩DIA = 0.7–0.9% is obtained with 3.6 MW of neutral beam injection heating at 0.94 T. The observed maximum diamagnetic beta is close to and slightly below the value predicted by theoretical MHD predictions. The correlation between the diamagnetic beta and observed MHD fluctuations is discussed. The paper includes a general technique used to analyse the diamagnetism of the plasma confined in a helical heliotron device.