A non-destructive gamma-ray spectrometry was applied to a number of volcanic products from Japan. Considerable amounts of 137Cs (∼10-13Ci/g) were determined in some volcanic rocks. The presence of 137Cs does not appear to have any relation to the type, age and spatial distribution of the volcanic rocks. This nuclide is not observed in new volcanic products such as ejecta which were collected immediately after eruption, while it is occasionally present in the interior of historic massive lavas. Based on the assumption that the source of the observed 137Cs is a fall-out, 239Pu in volcanic rocks is estimated to be ∼10-15Ci/g, being of the same order of magnitude as has been reported by MEIER et al. (1974).