  Two-stage spin-flop transitions are observed the in quasi-one-dimensional
antiferromagnet, BaCu${}_2$Si${}_2$O${}_7$. A magnetic field applied along the
easy axis induces a spin-flop transition at 2.0 T followed by a second
transition at 4.9 T. The magnetic susceptibility indicates the presence of
Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain
neighboring spins. We discuss a possible mechanism whereby the geometrical
competition between DM and interchain interactions, as discussed for the
two-dimensional antiferromagnet La${}_2$CuO${}_4$, causes the two-stage
spin-flop transitions.
