  We present 39 nights of optical photometry, 34 nights of infrared photometry,
and 4 nights of optical spectroscopy of the Type Ia SN 1999ac. This supernova
was discovered two weeks before maximum light, and observations were begun
shortly thereafter. At early times its spectra resembled the unusual SN 1999aa
and were characterized by very high velocities in the Ca II H and K lines, but
very low velocities in the Si II 6355 A line. The optical photometry showed a
slow rise to peak brightness but, quite peculiarly, was followed by a more
rapid decline from maximum. Thus, the B- and V-band light curves cannot be
characterized by a single stretch factor. We argue that the best measure of the
nature of this object is not the decline rate parameter Delta m_15 (B). The B-V
colors were unusual from 30 to 90 days after maximum light in that they evolved
to bluer values at a much slower rate than normal Type Ia supernovae. The
spectra and bolometric light curve indicate that this event was similar to the
spectroscopically peculiar slow decliner SN 1999aa.
