N2 fixation (C2H2 reduction) associated with the leaves of the sea grass Thalassia testudinum was investigated at 5 sites in South Florida (Biscayne Bay) and one site in the Bahamas (Bimini Harbor). Significant activities were correlated with the occurrence of a heterocystous blue-green alga (Calothrix sp.) on the leaves. C2H2 reduction was not stimulated by organic compounds, either aerobically or anaerobically in the light or dark. Therefore, other physiological types of microbes were not important in N2 fixation. Diurnal and seasonal variations in N2 fixation occurred, with maximal rates during the daytime and in the late spring and early summer. N2 fixation was negligible at four stations in Biscayne Bay. At the fifth station, near Fowey Rock, about 5 kg N ha-1 year-1 was fixed. In the summer, the N2 fixed per day (4–5 mg N m-2) could provide 4 to 23% of the foliar productivity demands of T. testudinum at this site and the station in Bimini Harbor. N2 fixation at the periphery of a sea-grass patch, near Fowey Rock, could provide 8 to 38% of the daily nitrogen requirement for leaf production, and thereby might compensate for a less effective trapping and recycling of nitrogen from dead leaves in such regions.