  The time evolution of barred structures is examined under the influence of
the external forces exerted by a spherical halo and by prolate halos. In
particular, galaxy disks are placed in the plane including the major axis of
prolate halos, whose configuration is often found in cosmological simulations.
N-body disks in fixed external halo fields are simulated, so that bars are
formed via dynamical instability. In the subsequent evolution, the bars in
prolate halos dissolve gradually with time, while the bar pattern in a
spherical halo remains almost unchanged to the end of the simulation. The decay
times of the bars suggest that they can be destroyed in a time smaller than a
Hubble time. Our results indicate that this dissolution process could occur in
real barred galaxies, if they are surrounded by massive dark prolate halos, and
the configuration adopted here is not unusual from the viewpoint of galaxy
formation. For a prolate halo model, an additional simulation that is
restricted to two-dimensional in-plane motions has also ended up with similar
bar dissolution. This means that the vertical motions of disk stars do not play
an essential role in the bar dissolution demonstrated here.
