  The filled skutterudite compound PrOs4Sb12, the first example of a Pr-based
heavy fermion superconductor, displays superconductivity with $T_c\sim 1.85$ K
and has an effective mass $m^* \sim$ {50 $m_e$}, where $m_e$ is the free
electron mass. For magnetic fields above 4.5 T, sharp features in the normal
state electrical resistivity, magnetization, specific heat, and thermal
expansion data suggest the occurrence of a phase transition at high fields.
This high field ordered phase in the normal state may originate from a
combination of crystalline electric field enhanced Zeeman splitting and
quadrupolar ordering. We present an investigation of the electrical resistivity
and magnetization of PrOs4Sb12 as a function of temperature between {350 mK}
and {3.5 K} and magnetic field up to {18 T}. The data reveal a detailed phase
boundary of the high field ordered phase as well as the lower critical field
$H_{c1}$ and the onset field of the peak effect in the superconducting state of
PrOs4Sb12.
