Direct evidence of heterotrophic dinitrogen fixation associated with the emergent aquatic angiosperm, Typha latifolia L., was obtained through the exposure of actively growing plants to 15N2 gas for 7 days in a gas-tight exposure vessel. Highest enrichments of 15N were found in roots/rhizomes and leaf bases. Slight enrichments were also found in the leaves due to translocation from the roots, rhizomes and leaf bases. Total fixed 15N values were 71.8 μg for the plant and 49.1 μg for the soil. Plants growing in silica sand, which received a nutrient solution containing combined nitrogen, exhibited higher enrichments and fixed 86% more 15N after exposure to 15N2 gas than plants which received a nutrient solution lacking combined nitrogen. It is hypothesized that the concentration of combined nitrogen added was insufficient to repress nitrogen fixation and resulted in an increase in nitrogen fixation by associated microorganisms. Propane was used to trace the loss and movement of gases from the 15N2 vessel and between the upper leaf chamber and the lower root chamber. Gas was rapidly exchanged between the upper and lower chambers through the leaves and roots of T. latifolia. Further investigations showed that propane moved at a rate of 1223 μmol day-1 from the leaves to the roots and 2652 μmol day-1 from the roots to the leaves. These data demonstrated that gases diffuse rapidly through the plant body of T. latifolia.