For the first time the seasonal cycle of optical and physical particle properties over the Indian Ocean was observed with a six-wavelength aerosol lidar. The measurements were performed in the framework of the Indian Ocean Experiment at the Maldives International Airport in Hulule island (4.1 ¿N, 73.3 ¿E) from February 1999 until March 2000. The advection of air masses from India and Southeast Asia during the northeast monsoon season showed multiple particle layers with strong backscatter and extinction coefficients to heights of 4 km. Approximately 30--45% of the monthly-mean optical depth was contributed by elevated particle layers above 1000 m height. Mean optical depths of ~0.32 at 530 nm and mean extinction-to-backscatter (lidar) ratios from 45 to 75 sr at 532 nm indicated heavily polluted aerosols. Observations during March 2000 showed that 56% of the mean optical depth of 0.29 was contributed by elevated particle layers. Lidar ratios between 45 and 60 sr were in the same range as in the previous year. The southwest monsoon season was characterized by the advection of particles in maximum heights of 5000 m from Africa and Arabia. Backscatter coefficients were considerably less compared to the findings during the winter monsoon. Low mean optical depths of 0.15, with a contribution of 48% from layers above 1000 m, and lidar ratios from 35--55 sr indicated a mixture of clean-marine and clean-continental aerosol conditions. During the intermonsoon season, clean-marine conditions prevailed with particles to maximum heights of 2.5 km, mean optical depths of 0.13, and lidar ratios from 20 to 30 sr. ¿ 2001 American Geophysical Union