Primary succession is controlled by a combination of landscape and habitat factors whose actions may be stochastic or deterministic. The 1980 eruption of Mount St. Helens, Washington spawned a massive lahar that now supports a mosaic of vegetation. Our goals were to describe vegetation patterns after 28 growing seasons, determine the factors associated with these patterns, and to contrast the effects of stochastic and deterministic processes. We described species composition and explanatory factors that included location and habitat features in one hundred and fifty one 200-m(2) plots. We classified these plots into nine community types (CTs) that were distinguished quantitatively by variations in dominant species. We used multiple regressions, redundancy analysis (RDA), and Mantel tests to compare the vegetation relationships with explanatory factors. Plots in different CTs mingled spatially and in multivariate space. Species patterns were weakly related to explanatory variables by RDA (31.6% of the species variation). RDA indicated that vegetation was most strongly related to elevation, latitude, and isolation, which are primarily landscape factors. Mantel tests confirmed that factors associated with elevation were most closely associated with vegetation. The effects of arrival order were suggested by the dominance of different colonizers in similar environment and by plots with similar vegetation found in different habitats. We concluded that species composition cannot be predicted well from the data available, suggesting that there were no prominent deterministic assembly rules.