  We investigate the temperature dependence of the lower critical field Hc1(T)
of a high-quality FeSe single crystal under static magnetic fields H parallel
to the c axis. The temperature dependence of the first vortex penetration field
has been experimentally obtained by two independent methods and the
corresponding Hc1(T) was deduced by taking into account demagnetization
factors. A pronounced change in the Hc1(T) curvature is observed, which is
attributed to anisotopic s-wave or multiband superconductivity. The London
penetration depth {\lambda}ab(T) calculated from the lower critical field does
not follow an exponential behavior at low temperatures, as it would be expected
for a fully gapped clean s-wave superconductor. Using either a two-band model
with s-wave-like gaps of magnitudes ?delta 1 = 0.41 meV and delta ?2 = 3.33 meV
or a single anisotropic s-wave order parameter, the temperature dependence of
the lower critical field Hc1(T) can be well described. These observations
clearly show that the superconducting energy gap in FeSe is nodeless.
