We have measured 3He/4He and 4He/20Ne ratios of gas samples collected at Lake Nyos where a toxic gas disaster occurred on August 21, 1986 and at lake Monoun in Cameroon, West Africa. The 3He/4He ratios, corrected for air contamination, are 5.39--5.75 Ratm (where Ratm is the atmospheric ratio of 1.40¿10-6) for Lake Nyos and 3.56 Ratm for Lake Monoun. The data suggest a significant contribution of mantle-derived helium in these Cameroon volcanic crater lakes. The origin of CO2 gas ''erupted'' during Lake Nyos disaster is inferred to be of magmatic origin based on the high 3He/4He ratio. ¿ American Geophysical Union 1987