  We report on the first detection of a quasi-simultaneous radio-X-ray flare of
Cygnus X-1. The detection was made on 2005 April 16 with pointed observations
by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where
the black hole candidate was close to a transition from the its soft into its
hard state. The radio flare lagged the X-rays by approximately 7 minutes,
peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag
in the context of models explaining such flaring events as the ejection of
electron bubbles emitting synchrotron radiation.
