Aerosol number size distributions were measured aboard the R/V Ronald H. Brown during the Indian Ocean Experiment (INDOEX) 1999 Intensive Field Phase (IFP) using a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS), covering a size range from 0.02 to 7 ¿m geometric diameter at 55% relative humidity (RH). The Indian Ocean marine boundary layer (MBL) aerosol number size distributions measured during the 1999 IFP were categorized into eight air mass source regions based on air mass back trajectories. The number and volume size distributions in these eight regions were distinctly different as a result of the different aerosol sources, meteorological conditions during transport, and time spent in the MBL. The aerosol sampling and data reduction during INDOEX were similar to that used during the Aerosol Characterization Experiment (ACE)-1 (Mid-Pacific Ocean 37¿N to 32¿S and Southern Ocean south of Tasmania, Australia), ACE-2 (North Atlantic Ocean west of Portugal and North Africa), and Aerosols99 (Atlantic Ocean transit from Norfolk, USA to Cape Town, South Africa) thus facilitating comparisons of the number and volume size distributions from these different experiments. The combined data set, summarized in this paper, provides regional aerosol parameters for comparison with global climate models and satellite retrieval algorithms.