We conducted field and culture experiments to study the effects of arbuscular mycorrhizal (AM) fungi on the establishment of the alien plant Oenothera laciniata (Onagraceae) in a coastal sand dune in Japan. We examined the distribution of plants, AM fungal colonization of their roots, and fungal spore density in the soil of a coastal sand dune at Keinomatsubara, Hyogo Prefecture, western Japan. Of the nine dominant plant species at the study site, six species, including O. laciniata, were heavily colonized by AM fungi. The spore density tended to decrease from inland toward the shoreline, but there were no significant relationships between the level of AM fungal colonization and spore density. Seedlings of O. laciniata inoculated with AM fungi, as well as uninoculated seedlings, were transplanted to the study site. No significant difference in survival rates was detected between the two groups of seedlings. We also examined the effects of AM infection and nutrient (N and P) application on plant growth in a pot culture experiment. The results suggested that the growth of O. laciniata was limited mainly by N availability and that AM mycorrhizae had little effect on the establishment of the plant on the coastal sand dune.