We have developed a mid-infrared continuous-wave quantum cascade laser direct-absorption spectrometer (QCLS) capable of high frequency (>= 1 Hz) measurements of (CH4)-C-12 and (CH4)-C-13 isotopologues of methane (CH4) with in situ 1-s RMS delta C-13(CH4) precision of 1.5 parts per thousand and Allan-minimum precision of 0.2 parts per thousand. We deployed this QCLS in a well-studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 +/- 0.7 mg CH4 m(-2) hr(-1) over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 +/- 0.8 mgCH(4) m(-2) hr(-1)) over the same period. Mean delta C-13(CH4) composition of emitted CH4 calculated using EC isoflux methods was -71 +/- 8 parts per thousand (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of -64.5 +/- 0.8 parts per thousand Ebullitive fluxes, representing similar to 10% of total CH4 fluxes at this site, were on average 1.2 parts per thousand enriched in C-13 compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process-based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.