  We report late-time observations of the radio afterglow of GRB 030329. The
light curves show a clear achromatic flattening at 50 days after the explosion.
We interpret this flattening as resulting from the blast wave becoming
trans-relativistic. Modeling of this transition enables us to make estimates of
the energy content of the burst, regardless of the initial jet structure or the
distribution of initial Lorentz factors of the ejecta. We find, in accordance
with other events, that GRB 030329 is well-described by an explosion with total
energy of a few times 10^51 erg expanding into a circumburst medium with a
density of order unity.
