Geotechnical properties and behavior are described of the very wet, halloysitic, residual clay from the dense rain forest at Monasavu, Viti Levu, Fiji, where the annual rainfall can exceed 5 m. The tropical climate has caused deep weathering of sandstones and produced a highly plastic clay with low density and a natural moisture content greatly in excess of the standard compaction optimum. This clay was found to contain halloysite which was "amorphous" rather than crystalline. The material was used in this natural state in an 85-m high rockfill dam at Monasavu Falls as an unusually soft core, the construction of which involved unconventionally light compaction by low-ground-pressure-tracked dozers. Its resulting behavior in terms of three-dimensional total and effective stresses, stress paths and deformations throughout the construction, impounding and full reservoir stages was closely monitored. This behavior is examined in the light of the clay's classification, minerlogical, compaction and engineering properties determined before and during construction. Despite its unusual properties, the clay is a good engineering material, behaving like others containing halloysite in the more common tubular form, and the high natural moisture content is of positive benefit.--Modified journal abstract.