Adaptation occurs by natural selection favoring traits that maximize fitness in a given environment. Interactions between species may influence selection, and the influence may depend on environmental conditions. The interaction between arbuscular mycorrhizal (AM) fungi and plants is likely to influence selection on resource-related traits as it involves fungi providing nutrients for the plant in exchange for sugars from photosynthesis. AM fungi are most beneficial to plants in low nutrient soils and so their influence on selection may be strongest in this context. I tested for the effect of inoculation with AM fungi and soil phosphorus level on selection for shoot phosphorus and photosynthesis by growing Lobelia siphilitica with and without AM fungi in low and high phosphorus soils. The results indicate that AM fungi increase plant fitness most in poor nutrient conditions, and that despite important trait responses the effect of the symbiosis on trait selection is minimal.