"THIS CASE WAS MODIFIED DECEMBER 10, 2009." The pilot performed a successful landing, about 30 minutes after sunset at the destination airport. He taxied off the runway onto a perpendicular taxiway, and was instructed by the ground controller to "taxi to the ramp." The pilot stated he was "unable to make out any features on the ground," due to the darkness. He observed the Fixed Base Operator (FBO) directly in front of him and proceeded straight ahead, not realizing there was a strip of grass separating the taxiway and the FBO. The airplane exited the taxiway and continued down a grassy slope between the ramp and taxiway. During the excursion from the taxiway, the propeller and the tail of the airplane struck the ground, resulting in substantial damage. The pilot did not report any mechanical malfunctions with the airplane. He said that the accident could have been prevented if he had studied the airport diagram more closely and noticed the grass strip.