  The thermal conductivity \kappa in the basal plane of single-crystalline
hexagonal NbSe2 has been measured as a function of magnetic field H, oriented
both along and perpendicular to the c-axis, at several temperatures below T_c.
With the magnetic field in the basal plane and oriented parallel to the heat
flux we observed, in fields well below H_c2, an unexpected hysteretic behavior
of \kappa(H) with all the generic features of a first order phase transition.
The transition is not manifest in the \kappa(H) curves, if H is still in the
basal plane but oriented perpendicularly to the heat-flux direction. The origin
of the transition is not yet understood.
