  The magnetic flux structure in the basal plane, (001), of single crystals of
superconducting (R = Er) and non-superconducting (R = Tb) RNi$_2$B$_2$C was
studied by high resolution Bitter decoration at temperatures below $T_c$
(superconducting transition) and/or $T_N$ (antiferromagnetic transition). For
both materials two sets of domain boundaries, in \{110\} and \{100\} planes,
were observed. The temperature ranges in which the \{100\} domain boundaries
were observed in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C coincide with the weak
ferromagnetic (WFM) ordering in these materials. On the other hand, the \{110\}
twin boundaries - the antiferromagnetic domain boundaries - were observed in
both compounds below $T_N$. The possibility of interpretation of \{100\}
boundaries as Bloch domain walls in the weakly ferromagnetic phase, for $T <
T_{WFM} < T_N$ (TbNi$_2$B$_2$C) or $T < T_{WFM} < T_N < T_c$ (ErNi$_2$B$_2$C)
is discussed.
