The presence of arbuscular mycorrhizas in fens has received little attention, but because fen plants are often phosphorus limited, the plant-fungus interaction could be an important factor in plant competition for phosphorus. In this field study, we determined mycorrhizal colonization rates for 18 fen plant species. Also in the field, we examined the effect of four different forms of phosphorus on the percentage colonization for one fen plant species, Solidago patula. We found that in a species-rich, phosphorus-poor wetland both mycorrhizal and nonmycorrhizal species were common. Nine of ten dicotyledonous species examined formed arbuscular mycorrhizas, while all monocotyledonous species were at most very weakly mycorrhizal. A morphological explanation for this pattern is that the monocots in our study have more extensive aerenchyma, especially in coarse roots. Therefore, monocots are able to transport oxygen to their roots more effectively than dicots. In the organic wetland soil, additional oxygen in the rhizosphere promotes phosphorus mineralization and availability. Two of the monocot species (Typha latifolia and Carex lasiocarpa), which have been described previously as mycorrhizal in other wetland types, are surprisingly nonmycorrhizal in our phosphorus-poor study site, suggesting that a mycorrhizal association would not offer improved phosphorus nutrition to these species. In contrast, our field phosphorus addition decreased mycorrhizal colonization in S. patula, suggesting that one benefit to S. patula of the mycorrhizas is phosphorus uptake.