  Resistivity measurements of a few-layer black phosphorus (bP) crystal in
parallel magnetic fields up to 45 T are reported as a function of the angle
between the in-plane field and the source-drain (S-D) axis of the device. The
crystallographic directions of the bP crystal were determined by Raman
spectroscopy, with the zigzag axis found within 5{\deg} of the S-D axis, and
the armchair axis in the orthogonal planar direction. A transverse
magneto-resistance (TMR) as well as a classically-forbidden longitudinal
magneto-resistance (LMR) are observed. Both are found to be strongly
anisotropic and non-monotonic with increasing in-plane field. Surprisingly, the
relative magnitude (in %) of the positive LMR is larger than the TMR above
$\sim$32 T. Considering the known anisotropy of bP whose zigzag and armchair
effective masses differ by a factor of approximately seven, our experiment
strongly suggests this LMR to be a consequence of the anisotropic Fermi surface
of bP.
