  We present temperature-dependent in-plane resistivity measurements on FeSe
single crystals under He-gas pressure up to 800 MPa and magnetic fields $B
\leq$ 10 T. A sharp phase transition anomaly is revealed at the
tetragonal-to-orthorhombic transition at $T_s$ slightly below 90 K. $T_s$
becomes reduced with increasing pressure in a linear fashion at a rate
d$T_{s}$/d$P$ $\simeq$ -31 K/GPa. This is accompanied by a $P$-linear increase
of the superconducting transition temperature at $T_c \sim$ 8.6 K with
d$T_{c}$/d$P$ $\simeq$ +5.8 K/GPa. Pressure studies of the normal-state
resistivity highlight two distinctly different regimes: for $T > T_s$, i.e., in
the tetragonal phase, the in-plane resistivity changes strongly with pressure.
This contrasts with the state deep in the orthorhombic phase at $T \ll T_s$,
preceding the superconducting transition. Here a $T$-linear resistivity is
observed the slope of which does not change with pressure. Resistivity studies
in varying magnetic fields both at ambient and finite pressure reveal clear
changes of the magnetoresistance, $\Delta \rho \propto B^{2}$, upon cooling
through $T_s$. Our data are consistent with a reconstruction of the Fermi
surface accompanying the structural transition.

Superconductivity