  Our purpose is to place firm observational constraints on the three most
widely used theoretical models for the spatial configuration of the large-scale
interstellar magnetic field in the Galactic disk, namely, the ring, the
axisymmetric and the bisymmetric field models.
  We use the rotation measures (RMs) of low-latitude Galactic pulsars and
combine them with their dispersion measures and estimated distances to map out
the line-of-sight component of the interstellar magnetic field in the near half
of the Galactic disk. We then fit our map of the line-of-sight field to the
three aforementioned theoretical field models and discuss the acceptability of
each fit, in order to determine whether the considered field model is allowed
by the pulsar data or not.
  Strictly speaking, we find that all three field models are ruled out by the
pulsar data. Furthermore, none of them appears to perform significantly better
than the others. From this we conclude that the large-scale interstellar
magnetic field in the Galactic disk has a more complex pattern than just
circular, axisymmetric or bisymmetric.
