[en] Monazite, a thorium-bearing mineral, has been identified in the heavy mineral fractions of the fine sands from the alluvial soil profiles at Barpalia and Gyaspur of Siwan district in north Bihar. The amount of monazite varied from 14.29 to 15.40 per cent and 6.25 to 9.38 per cent of the heavy minerals in the fine sands of Barpalia and Gyaspur, respectively. Monazite could have been derived from the granite pegmatite and gneissic metamorphic rocks in association with feldspar, zircon and apatite from the Himalayan orogan. The amount of ²³²Th present in monazite originally ranges from a few wt per cent ThO₂ to 10-12 per cent. Even in the east coast of India , monazite contains 9.05 per cent ThO₂. (author).5 refs., 2 tabs., 1 fig