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 \tilde { r } _ { E } \approx 29.2 AU .
The transverse velocity projected onto the observer plane is about 79 { km s ^ { -1 } } .
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 .