  We report the first detection of CO(1-0) emission from a
submillimeter-selected galaxy, using the Green Bank Telescope. We identify the
line in the spectrum of SMM J13120+4242 as a broad emission feature at z=3.408,
with Delta(V_FWHM)=1040 +/- 190 km/s. If the observed CO(1-0) line profile
arises from a single object and not several merging objects, then the
CO(4-3)/CO(1-0) brightness temperature ratio of ~0.26 suggests n(H_2) > 3-10 x
10^2 cm^-3 and the presence of sub-thermally excited gas. The integrated line
flux implies a cold molecular gas mass M(H_2)=1.6 x 10^11 M_sun, comparable to
the dynamical mass estimate and 4 times larger than the H_2 mass predicted from
the CO(4-3) line assuming a brightness temperature ratio of 1.0. While our
observations confirm that this submillimeter galaxy is massive and gas-rich,
they also suggest that extrapolating gas masses from J_upper >= 3 transitions
of CO leads to considerable uncertainties. We also report an upper limit to the
mass of cold molecular gas in a second submillimeter galaxy, SMM J09431+4700,
of M(H_2)< 4 x 10^10 M_sun.
