Differences in vibration magnitude required for a human subject to differentiate whole-body vertical sinusoidal vibrations, difference thresholds for amplitude of sinusoidal vibration, have been determined at a vibration magnitude of 0.7 m/s2 r.m.s. at five octave band center frequencies from 4 to 63 Hz and at 80 Hz. The median difference thresholds of 16 male subjects seated on a flat rigid seat were found between 0.037 and 0.046 m/s2 r.m.s. at the frequencies used in this study. The subjects tended to be more sensitive to the change in vibration magnitude at 4 Hz than at 16, 31.5 and 63 Hz and less sensitive to the magnitude difference at 31.5 Hz than at 4, 8 and 80 Hz. The median relative difference thresholds, Weber's ratios, varied from 5.2% to 6.5% which were lower compared to the relative difference thresholds determined in the previous studies at frequencies where comparable data were available. The causes of the difference in the relative difference thresholds observed between this study and previous studies may include the difference in the psychophysical method used to determine the difference threshold.