  We describe the discovery of the longest microlensing event ever observed,
OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as
MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 641
days. The high quality data obtained with difference image analysis shows a
small but significant parallax signature. This parallax effect allows one to
determine the Einstein radius projected onto the observer plane as rE^hat ~
29.2AU. The transverse velocity projected onto the observer plane is about
79km/s. We argue that the lens is likely to be have a mass of at least a few
solar masses, i.e., it could be a stellar black hole. The black hole hypothesis
can be tested using the astrometric microlensing signature with the soon-to-be
installed Advanced Camera for Surveys on board the Hubble Space Telescope. Deep
X-ray and radio images may also be useful for revealing the nature of the
object.
