How vegetation develops from a source of potential species remains poorly understood. I explored whether colonizing species assemble randomly, or if local deterministic factors alter species establishment to create consistent vegetation patterns. Early primary successional vegetation in 111 small self-contained depressions (potholes) on Mount St. Helens was sampled. Mean richness was 8.8 species and mean cover was 2.6%. Mean percent similarity (PS) between potholes was 46%. A nearby grid of 100 contiguous 100 m² quadrats on a level barren plain was sampled for comparison. Barren quadrats had mean richness of 12.2 species, mean cover of 1.4% and mean PS of 63%. Pothole vegetation was much more variable than that of the barrens. Canonical correspondence analysis revealed a slight relationship between environmental factors and pothole vegetation. Species composition, cover and species rank orders were predicted extremely well by a stochastic model based on observed frequencies and random accumulation of species. These findings suggest that early colonization of isolated sites is stochastic, and that dispersal affects species composition more strongly than environmental factors or competitive interactions. Which species succeed at any particular site is poorly predictable, but once established, a local population can persist. As a result, there is often a poor correlation between species composition and environmental factors in mature vegetation.