  We report on the observation of dendritic flux avalanches in a large Niobium
single crystal. In contrast to avalanches observed in thin films, they appear
only in a very narrow temperature interval of about a tenth of a Kelvin near
the critical temperature of Nb. At a fixed temperature, we find two sets of
dendritic structures, which differ by the magnetic field required for their
formation and by the maximum distance the dendrites penetrate into the sample.
The effect is caused by dendritic flux penetration into thin superconducting
surface layers formed in the single crystal close to the critical temperature.
