  We present multi-color optical and two-frequency radio observations of the
bright SAX event, GRB 990510. The well-sampled optical decay, together with the
radio observations are inconsistent with simple spherical afterglow models. The
achromatic optical steepening and the decay of the radio afterglow both
occuring at $t \sim 1$ day are evidence for hydrodynamical evolution of the
source, and can be most easily interpreted by models where the GRB ejecta are
collimated in a jet. Employing a simple jet model to interpret the
observations, we derive a jet opening angle of $\theta_o = 0.08$, reducing the
isotropic gamma-ray emission of $2.9 \times 10^{53}$ erg by a factor $\sim
300$. If the jet interpretation is correct, we conclude that GRB observations
to-date are consistent with an energy for the central source of $E \lsim
10^{52}$ erg.
