Ten years of isentropic trajectories were summarized using cluster analysis to describe flow patterns for American Samoa. The trajectories were then paired with surface ozone data to determine the dependence of surface ozone on transport. The two main transport regimes affecting surface ozone are trade wind transport, where trajectories show flow bringing ozone from the east in the tropical marine boundary layer, and midlatitude transport, where trajectories show strong westerly flow at higher elevations of southern midlatitudes, followed by descent with anticyclonic curvature. These two transport regimes yield ozone from distinctly different origins, having different mixing ratios. The seasonally changing frequency of transport type is shown to be partly responsible for the seasonal cycle and changes in variability of Samoa surface ozone. On average, 45% of winter ozone variation can be explained by differences in transport type. This strong relationship was absent, however, during 1991, probably because of UV blocking by aerosols from the eruption of Mount Pinatubo. Reduced total column ozone during winter 1992 may have contributed to this season having the lowest surface ozone levels of the study period.