We have determined from interferometry the radius of the nearby star tau ceti, using recent observations with the VINCI instrument on VLTI using the siderostats.

  The limb-darkened disk diameter is determined, with an unprecedented internal precision of 0.5%, to be 1.971+/-0.009_int+/-0.05_ext mas corresponding to a physical radius of 0.773+/-0.004_int+/-0.02_ext solar radii.

  With this determination tau ceti becomes a prime target for asteroseismic campaigns to determined its internal, and thereby test stellar evolution theory.