  Many spiral galaxies show a large-scale asymmetry with a cos\phi dependence
in their rotation curves as well as in their morphology, such as M101 and NGC
628. We show that both these features can be explained by the response of a
galactic disk to an imposed lopsided halo potential. A perturbation potential
of 5 % is deduced for the morphologically lopsided galaxies in the Rix &
Zaritsky (1995) sample. This is shown to result in a difference of 10 % or
about 20-30 kms^{-1} in the rotation velocity on the two sides of the major
axis. Interestingly, the observed isophotal asymmetry in a typical spiral
galaxy is not much smaller and it results in a velocity asymmetry of 7 % or
about 14-21 kms^{-1} . Hence, we predict that most galaxies show a fairly
significant rotational asymmetry. The rotation velocity is shown to be maximum
along the elongated isophote - in agreement with the observations along the SW
in M101, while it is minimum along the opposite direction. This result leads to
the distinctive asymmetric shape of the rotation curve which rises more steeply
in one half of the galaxy than the other, as observed by Swaters et al. (1999).
This shape is shown to be a robust feature and would result for any centrally
concentrated disk. The net disk lopsidedness and hence the asymmetry in the
rotation curve is predicted to increase with radius and hence can be best
studied using HI gas as the tracer.
