AFTER TAKEOFF, THE ULTRALIGHT VEHICLE WAS OBSERVED TO CLIMB ABOUT 600 TO 700 FT OVER A PASTURE EAST OF THE RWY, THEN THE PLT BEGAN TO PERFORM AEROBATIC MANEUVERS. AFTER COMPLETING A LOOP, THE ULTRALIGHT WAS OBSERVED TO ROTATE NOSE UP. AT ABOUT THAT TIME, A WITNESS HEARD A CRACKING SOUND. HE SAID THE ROTATION THEN STOPPED & THE ULTRALIGHT STARTED FALLING NOSE 1ST TOWARD THE GROUND. A PLT, WHO WAS WATCHING, SAID THE LEFT WING WAS FLUTTERING, BUT DID NOT COLLAPSE DURING THE UNCONTROLLED DESCENT. REPORTEDLY, THE ENG WAS AT FULL POWER UNTIL IMPACT. THE PREVIOUS OWNER, WHO WAS ALSO A DEALER, SAID HE HAD WARNED THE PLT NOT TO PERFORM AEROBATICS IN THIS ULTRALIGHT. AN EXAM OF THE WRECKAGE REVEALED THAT THE LEFT WING INNER & CENTER COMPRESSION STRUTS WERE BENT & THE LEFT TRAILING EDGE SPAR WAS BROKEN AT THE INNER COMPRESSION STRUT ATTACHING BOLT HOLE. THE INNER SURFACE OF THE REAR SPAR, NEAR THE COMPRESSION STRUT, SHOWED SIGNS OF COMPRESSION & COLUMN BUCKLING PRIOR TO FRACTURE.