  The I-Mn-V antiferromagnet, NaMnBi, develops a very large positive
magnetoresistance (MR) up to 10,000% at 2 K and 9 T in crystals showing a
semiconductor-to-metal transition (SMT). In the absence of an SMT, a modest
(20%) MR is achieved. Here, we show that upon cooling below the magnetic
transition, a spatial modulation appears giving rise to new Bragg peaks due to
charge and defect ordering in a checkerboard pattern, with two kinds of
modulation vectors, $q_1$=($\frac23$, 0, 1) and $q_2$=($\frac23, \frac13,
\frac12$). This constitutes a superlattice transition ($T_s$) that lowers the
symmetry from the high temperature centrosymmetric P4/nmm to the
non-centrosymmetric P$\overline4$m2. In crystals with a large MR, a close to
room temperature $T_s$ is observed with $q_1$ appearing first, followed by
$q_2$. In crystals with low MR however, $T_s$ is much lower and only $q_1$ is
observed. The charge modulation and spin fluctuations may both contribute to
the enhancement of MR.
