Results of a shadehouse experiment and a field study with seedlings and saplings of Inga leiocalycina Willd. (Leguminoseae), a canopy tree of American lowland tropical wet forests, indicated that plant light environment affects both (1) the degree of root colonization by vesicular-arbuscular mycorrhizal (VAM) fungi and (2) the development of VAM structures in the root. Among shadehouse-grown seedlings, VAM colonization increased with increasing light intensity, from a mean of 20 percent (21.5 cm colonized root) at a light level approximating forest understory environments to 47 percent (233.1 cm) at a light level found in centers of treefall gap environments. In contrast, VAM colonization levels were similar between gap and understory habitats among field-collected sapling roots; however, the anatomy of the symbiosis differed between light environments. Roots of light gap saplings had significantly more intracellular fungal coils than understory sapling roots. At subsaturating levels for photosynthesis, light availability appeared to be a limiting resource for this symbiosis, restricting the extent of VAM fungal colonization and development of symbiotic structures. Despite this light-mediated limitation, woody seedlings showed evidence of VAM fungi in 20 to 30 percent of their roots at light levels typical of the tropical wet forest ecosystem in general. I consider these results in the context of forest dynamics, and I suggest that, for woody seedlings, maintaining moderate levels of VAM in understory environments is of long-term benefit because it enables them to respond rapidly to the increased nutrient demand imposed by their increased growth rate when canopy openings occur.