  The total cross sections of charm and beauty production at a future
high-energy ep collider, THERA, are expected to increase by factors of three
and five, respectively, as compared to HERA. Heavy quarks can be measured at
THERA in wide ranges of transverse momenta and Q^2 values, thereby providing a
solid basis for testing the perturbative QCD calculations. The charm and beauty
contributions to the proton structure can be probed at THERA at ~1 order of
magnitude smaller Bjorken x values with respect to those at HERA. Charm
production in the process of photon-gluon fusion at THERA can serve for the
determination of the gluon structure of the proton in the as yet unexplored
kinematic range 10^{-5} < x_g < 10^{-4}. The cross section of charm production
in charged current at THERA is ~1 order of magnitude larger than that at HERA.
