Let M be an indefinite Finsler
space. The bisector of two points of M is the set of points equidistant from these two
points. A bisector is called flat if with any pair of points it contains the extremals
joining this pair. In this paper it is shown that M is pseudo-Riemannian of constant
curvature if and only if M locally has flat bisectors. Another result is that M is
pseudo-Riemannian of constant curvature if and only if M can be reflected locally in
each nonnull extremal.