The purpose of this work is to refine our knowledge about the nature of helium with a high abundance of the rare isotope3He(3He/4He= 10−5) discovered in terrestrial volcanic gases in 1968. We will discuss here the results of isotope analyses of helium released by step-wise heating of ultrabasic xenoliths and some volcanic rocks. On the basis of these results, possible sources of3He in the earth due to fission and nuclear reactions are considered critically. The most probable source of the high abundance of3He is shown to be due to the capture and trapping of primordial He by the earth during its formation (primordial helium3He/4He= 3 × 10−4), a small but significant fraction of which has been retained to the present time.