  We study the Rossby wave instability (RWI) in a thin accretion disc threaded
by an initially toroidal magnetic field using the magnetohydrodynamics (MHD)
code PLUTO. We find that for plasma $10^1 < \beta < 10^3$ , the growth rate and
late-time density are suppressed, whereas for plasma $\beta > 10^3$, the
magnetic field has negligible effect. The initially toroidal field is twisted
inside the vortex and expelled at late times. This creates a radially directed
field outside the vortex region, which can be understood via a simple kinematic
model of a magnetic field in a rotating fluid and may be observable via
polarised dust emission.
