Iron-rich red-brown earths, referred to generically as laterites, are not confined to the tropics but their greater frequency in the present day tropics provides some evidence and much cause for speculation concerning their origins. Prescott & Pendleton (1952) gave a detailed account of the composition and characteristics of laterites on a world-wide basis and Bruckner (1957) has suggested that the profiles of both laterites and bauxites in West Africa indicate that their origin is the direct consequence of weathering under rhythmic changes in the tropical climatic belt. Laterite is a soil, rich not only in iron but often also in aluminium, chromium and manganese, which has the property of hardening after exposure to air until it assumes a brick-like texture. In West Africa it is the product of chemical weathering of the Precambrian native bed-rocks, during which process the monobasic and dibasic components are leached away. Topographic features appear to be important in determining the depth of the laterite which forms in any one site. Of significance are the effects which seasonal changes in the depth at which the soil is saturated by ground water have within the lateritic soil, and also the relationship between rates of lateritization and erosion. In well-drained sites the continuous downward leaching process permits deep weathering of the bed-rock and progressive conversion of the lateritic overburden to laterite. In sites where there is a seasonal rise and fall of the water table, complex iron silicates become concentrated at the lower, dry season level. This leads to the formation of concretionary nodules (pisolites) and their cementation into an ironstone pan. In poorly drained sites where surface erosion is active, the weathering and cementation proceed in close sequence. Little or no lateritic earth accumulates and a superficial ironstone hardpan is formed, which has a tessellated structure and becomes areolar under waterlogged conditions. Where surface erosion is less active the presence of a pan renders the overburden of lateritic earth unsuitable for the support of a woody vegetation and, because lateritic soil dries out during the dry season, slow and progressive erosion of the exposed surface may lead to exposure of the ironstone hardpan. This account is of the vegetation on the exposed ironstone hardpan surfaces in three areas: around the Freetown peninsula, at Kambia (90 7' N, 120 55' W) and between Konta and Kukuna near the Guinea border (Fig. 1).