  We present a relationship between spiral arm pitch angle (a measure of the
tightness of spiral structure) and the mass of supermassive black holes (BHs)
in the nuclei of disk galaxies. We argue that this relationship is expected
through a combination of other relationships, whose existence has already been
demonstrated. The recent discovery of AGN in bulgeless disk galaxies suggests
that halo concentration or virial mass may be one of the determining factors in
BH mass. Taken together with the result that mass concentration seems to
determine spiral arm pitch angle, one would expect a relation to exist between
spiral arm pitch angle and supermassive BH mass in disk galaxies, and we find
that this is indeed the case. We conclude that this relationship may be
important for estimating evolution in BH masses in disk galaxies out to
intermediate redshifts, since regular spiral arm structure can be seen in
galaxies out to z~1.
