  We report on the discovery of the radio afterglow from the gamma-ray burst
(GRB) of 1999 May 6 (GRB 990506) using the Very Large Array (VLA). The radio
afterglow was detected at early times (1.5 days), but began to fade rapidly
sometime between 1 and 5 days after the burst. If we attribute the radio
emission to the forward shock from an expanding fireball, then this rapid onset
of the decay in the radio predicts that the corresponding optical transient
began to decay between 1 and 5 minutes after the burst. This could explain why
no optical transient for GRB 990506 was detected in spite of numerous searches.
The cause of the unusually rapid onset of the decay for the afterglow is
probably the result of an isotropically energetic fireball expanding into a low
density circumburst environment. At the location of the radio afterglow we find
a faint (R ~ 24 mag) host galaxy with a double morphology.
