  The magnetic ordering of La$_{1/3}$Sr$_{2/3}$FeO$_3$ perovskite has been
studied by neutron powder diffraction and $^{57}$Fe M\"ossbauer spectroscopy
down to 2 K. From symmetry analysis, a chiral helical model and a collinear
model are proposed to describe the magnetic structure. Both are commensurate,
with propagation vector k = (0,0,1) in R-3c space group. In the former model,
the magnetic moments of Fe adopt the magnetic space group P3$_{2}$21 and have
helical and antiferromagnetic ordering propagating along the c axis. The model
allows only one Fe site, with a magnetic moment of 3.46(2) $\mu_{\rm{B}}$ at 2
K. In the latter model, the magnetic moments of iron ions adopt the magnetic
space group C2/c or C2'/c' and are aligned collinearly. The model allows the
presence of two inequivalent Fe sites with magnetic moments of amplitude
3.26(3) $\mu_{\rm{B}}$ and 3.67(2) $\mu_{\rm{B}}$, respectively. The neutron
diffraction pattern is equally well fitted by either model. The M\"ossbauer
spectroscopy study suggests a single charge state Fe$^{3.66+}$ above the
magnetic transition and a charge disproportionation into Fe$^{(3.66-\zeta)+}$
and Fe$^{(3.66+2\zeta)+}$ below the magnetic transition. The compatibility of
the magnetic structure models with the M\"ossbauer spectroscopy results is
discussed.
