Measurements made of the concentration of aerosol particles in the diameter range 0.15--3 μm in the free troposphere over the southwest Pacific show evidence of a seasonal cycle. The maximum concentration occurs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30¿S where, in early spring, the mean aerosol concentration is 18 cm-3, more than 10 times the mean autumn value. In the vicinity of the equator the seasonality disappears and concentrations throughout the year apparently remain close to the autumn levels at other latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that the peak in early spring is due to biomass burning through the tropical southern hemisphere dry season. Continued high values through late spring and early summer are thought to be the result of the seasonal production of aerosol precursors, such as dimethyl sulfide (DMS), at middle and high latitudes. ¿ American Geophysical Union 1993