  We present rest-frame optical and near-infrared detections of one of the
highest redshift submm-selected galaxies to date, HDF850.1. We do not detect
the previously proposed counterpart HDF850.1K in new deep J and H-band HST WFC3
data, placing a strong limit of H-K>3.8, concluding that the K-band source is
spurious. However, we detect 5.8um and 8um emission co-located with the submm
in deblended images. After modelling and removing the flux contributions from
another foreground galaxy, we constrain the stellar mass of HDF850.1 to be
(2.5+/-1)x10^{11} Msun/mu for a lensing magnification mu=1.9+/-0.3, with a
specific star formation rate of 8.5 Gyr^{-1}, faster than the 1-4 Gyr^{-1}
observed for UV-selected galaxies at this epoch.
