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We have detected the CO(2–1) transition from the submillimetre galaxy (SMG) LESS J 275619 at using the new Compact Array Broadband Backend system on the Australian Telescope Compact Array. These data have identified a massive gas reservoir available for star formation for the first time in an SMG at . We use the luminosity and velocity width (full width at half-maximum, FWHM, of km s−1) of the CO(2–1) line emission to constrain the gas and dynamical mass of and (0.25/sin ) M , respectively, similar to that observed for SMGs at lower redshifts of , although we note that our observed CO FWHM is a factor of ∼3 narrower than typically seen in SMGs. Together with the stellar mass we estimate a total baryonic mass of , consistent with the dynamical mass for this young galaxy within the uncertainties. Dynamical and baryonic mass limits of high-redshift galaxies are useful tests of galaxy formation models: using the known SMGs as examples of massive baryonic systems, we find that their space density is consistent with that predicted by current galaxy formation models. In addition, these observations have helped to confirm that SMGs possess the baryonic masses and gas consumption time-scales necessary to be the progenitors of the luminous old red galaxies seen at . Our results provide a preview of the science that ALMA will enable on the formation and evolution of the earliest massive galaxies in the Universe. [ABSTRACT FROM AUTHOR]