Samples collected from the deep Indian Ocean on the GEOSECS Indian Ocean Expedition and R.V. Vema cruise 34 reveal that horizontal processes control mixing in the lower 1 km of the water column. Intermediate 228Ra maxima occur throughout the deep water of the Indian Ocean. 228Ra becomes enriched in waters in contact with bottom sediments. This probably occurs as horizontally migrating water masses move from one basin to another. Thus 228Ra seems to trace the flow of water through the deep Indian Ocean. In the basins of the northwest Indian Ocean (Arabian, Madagascar and Crozet), an intermediate 228Ra maximum is associated with high Si waters originating in the north. A broad 228Ra maximum in the Wharton Basin is probably derived from a northward flowing western boundary current from the Antarctic. In the Central Indian Basin high Si waters spreading southward from the Bay of Bengal do not carry a significant 228Ra signal; but water from the south, which overrides this high Si water occurring at about 3700 m depth, does carry a significant 228Ra signal above and below 3700 m.