  The thermal conductivity of the iron-based superconductor FeSe was measured
at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic
field, the electronic residual linear term in the T = 0 limit, \kappa_0/T, is
vanishingly small. Application of a magnetic field H causes no increase in
\kappa_0/T initially. Those two observations show that there are no zero-energy
quasiparticles that carry heat and therefore no nodes in the superconducting
gap of FeSe. The full field dependence of \kappa_0/T has the classic shape of a
two-band superconductor, such as MgB2: it rises exponentially at very low
field, with a characteristic field H* << Hc2, and then more slowly up to the
upper critical field Hc2. This shows that the superconducting gap is very small
on one of the pockets in the Fermi surface of FeSe.
