We present multi-color optical and two-frequency radio observations of the bright SAX 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 , 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 explain the observations , we derive a jet opening angle of \theta _ { o } = 0.08 ( n / 1 cm ^ { -3 } ) ^ { 1 / 8 } , reducing the isotropic gamma-ray energy release of 2.9 \times 10 ^ { 53 } erg by a factor \sim 300 .