The submicron aerosol number size distributions are measured over the Indian Ocean (15¿N, 77¿E to 20¿S, 58¿E) during the Indian Ocean Experiment (INDOEX) in winter monsoon seasons of 1998 and 1999. Large concentrations of 4 ¿ 102 and 104 cm-3 are observed with comparatively much higher values toward north than south of the Intertropical Convergence Zone (ITCZ). The nucleation mode in the aerosol size distributions shows large variability, especially north of the ITCZ. The monomodal size distributions of particles observed close to coastline gradually become more and more organized into bimodal size distributions as one proceeds from north to south. The shift of the position of the minimum in size distributions to larger particle size as one proceeds southward suggests the growth of nucleation mode particles by homogeneous or heterogeneous processes. Observation of the north-to-south increase in the accumulation mode particles indicates the removal of these particles by sedimentation or cloud droplet nucleation and precipitation scavenging as the aerosol particles are advected southward. The bimodal size distributions mostly show a maximum at 133 nm and a minimum at 42 nm and are generally open-ended at the small-size end, indicating a maximum for ultrafine particles of <13 nm diameter. There are strong indications that the nucleation mode particles formed in the free troposphere by the gas-to-particle conversion processes in the outflow regions of clouds of the convective cells are transported down to the marine boundary layer in the descending leg of Hadley cell in the Northern Hemisphere and then advected southward along with the continental aerosols with the persistent northeasterly surface winds in this season. A high peak of 13 nm particles exceeding 103 particles cm-3 is associated with the mesoscale subsidence of a depression.