Stable hydrogen (delta D) and carbon (delta(13)C) isotope ratios of sedimentary methane from five subtropical Florida freshwater sites exhibited smaller, less distinct seasonal variations than previously observed in temperate sediments, apparently due to the smaller range of temperatures forcing the subtropical environments. Negative correlations observed between (delta D)-CH4 and (delta(13)C)-CH4 at the sites probably resulted from mixing methane produced via acetate fermentation with methane formed from CO2 and H2O. The inverse (delta D)/(delta(13)C) trends, high sedimentary gas methane concentrations (up to 80 mole%), and visual observations imply that ebullition dominates methane transport from the sediments during at least part of the seasonal cycle. Many values deviated from the main inverse delta D/delta(13)C trend such that bacterial oxidation was not indicated. The deviation may result from the reduction of CO2 with varying delta(13)C under conditions of elevated H2 concentrations or enhanced interspecies hydrogen transfer, which may cause relatively greater hydrogen isotopic fractionation. Alternatively, the deviation may result from fermentation of autotrophically formed, (13)C-depleted acetate. Differences in the delta(13)C of bulk organic matter from two of the sites potentially account for about half of the difference in mean delta(13)C-CH4 between the two sites. In other cases, however, the immediate methanogenic precursor appeared to be the major determinant of methane stable isotopic composition. (Copyright (c) 1992 by the American Geophysical Union.)