  We report a neutron diffraction study of single-crystal CePtAl$_3$
complemented by measurements of the specific heat under applied magnetic field.
Below $T_\mathrm{N}$=3 K CePtAl$_3$ develops incommensurate antiferromagnetic
order with a single modulation vector $\vec{k}$=$(0.676 \, 0 \, 0)$. Residual
magnetic scattering intensity above $T_\mathrm{N}$ and a broadening of the
specific heat anomaly at $T_\mathrm{N}$ may be consistently described in terms
of a Gaussian distribution of transition temperatures with a standard deviation
$\sigma\approx0.5\,{\rm K}$. The distribution of $T_\mathrm{N}$ may be
attributed to the observation of occupational and positional disorder between
the Pt and Al sites. Measurements under magnetic field reveal unusual changes
of the domain populations when the field is applied along the $[0\,1\,0]$
direction consistent with a transition from cycloidal to amplitude modulated
magnetic order around 2.5 T.
