The &dgr;13C value of pore water methane produced in a Michigan peatland varied by 11? during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the &dgr;13C value of methane from CO2 reduction (average=-71.4¿1.8?) was depleted in 13C compared to that produced from acetate (-44.4¿8.2?). The dissolved methane at the site remained heavy (approximately -51?) during most of the year. Tracer experiments using 14C-labeled CO2 indicated that during January 110¿25% of the methane was produced by CO2 reduction. Because of low-methane production rates during the winter, this 13C-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the &dgr;13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached -61?. Peat incubation experiments conducted at 15¿C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84¿9% of the methane production was from acetate and had an average &dgr;13C value of -48.7¿5.6?. Rising acetate concentrations during April-May (approaching 1 mmolL-1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately -51?) similar to that produced in the incubation experiments. ¿ 1999 American Geophysical Union