  We report measurements of the in-plane magnetic penetration depth $\Delta
\lambda $(T) in single crystals of ErNi$_{2}$B$_{2}$C down to $\sim$0.1 K using
a tunnel-diode based, self-inductive technique at 21 MHz. We observe four
features: (1) a slight dip in $\Delta \lambda $(T) at the N$\acute{e}$el
temperature $T_{N}$ = 6.0 K, (2) a peak at $T_{WFM}$ = 2.3 K, where a weak
ferromagnetic component sets in, (3) another maximum at 0.45 K, and (4) a final
broad drop down to 0.1 K. Converting to superfluid density $\rho_{s}$, we see
that the antiferromagnetic order at 6 K only slightly depresses
superconductivity. We seek to explain some of the above features in the context
of antiferromagnetic superconductors, where competition between the
antiferromagnetic molecular field and spin fluctuation scattering determines
increased or decreased pairbreaking. Superfluid density data show only a slight
decrease in pair density in the vicinity of the 2.3 K feature, thus supporting
other evidences against bulk ferromagnetism in this temperature range.
