  We report on detailed experimental studies of a high-quality heterojunction
insulated-gate field-effect transistor (HIGFET) to probe the particle-hole
symmetry (PHS) of the FQHE states about half-filling in the lowest Landau
level. The HIGFET was specially designed to vary the density of a
two-dimensional electronic system under constant magnetic fields. We find in
our constant magnetic field, variable density measurements that the sequence of
FQHE states at filling factors nu = 1/3, 2/5, 3/7 ... and its particle-hole
conjugate states at filling factors 1 - nu = 2/3, 3/5, 4/7 ... have a very
similar energy gap. Moreover, a reflection symmetry can be established in the
magnetoconductivities between the nu and 1 - nu states about half-filling. Our
results demonstrate that the FQHE states in the lowest Landau level are
manifestly particle-hole symmetric.
