We present multicolor optical and two-frequency radio observations of the bright BeppoSAX event GRB 990510. Neither the well-sampled optical decay nor the radio observations are consistent with simple spherical afterglow models. The achromatic steepening in the optical band and the early decay of the radio afterglow, which both occur at tåisebox-0.5ex 1 day, are evidence for hydrodynamical evolution of the source and can be most easily interpreted by models in which the gamma-ray burst ejecta are collimated in a jet. Employing a simple jet model to explain the observations, we derive a jet opening angle of þeta$_0$=0.08(n/1 cm$^-3$)$^1/8$, reducing the isotropic gamma-ray energy release of 2.9×10$^53$ ergs by a factor of i̊sebox-0.5ex 300.