Atmospheric concentrations of aerosol methanesulfonic acid (MSA) and non-sea-salt (nss) sulfate were measured in samples collected during one year periods at Fanning and American Samoa Islands in the Pacific Ocean. Mean concentrations were 0.045 (s=0.011) ug/m3 MSA and 0.67 (s=0.13) ug/m3 nss sulfate at Fanning, and 0.026 (s=0.012) ug/m3 MSA and 0.41 (0.19) ug/m3 nss sulfate at American Samoa. MSA and nss sulfate were significantly correlated at both stations (r=0.76), with a mean MSA/nss sulfate ratio of 0.065. The higher mean levels of MSA and nss sulfate at Fanning compared to American Samoa apparently reflect differences in the biological productivity of the surface ocean in the two regions.