ON 5/25/07, EMPLOYEE WAS WALKING FROM RK#1 & #2 TO PICK UP THE 3RD SAMPLE. JUST BEFORE SHE GOT TO THE RAIL CAR TRACKS, SHE STEPPED ON A STONE AND STUMBLED BUT DID NOT FALL. SHE FELT PAIN IMMEDIATELY IN HER LEFT ANKLE. SHE WAS ABLE TO FINISH HER ROUNDS. SHE REPORTED THE INCIDENT TO THE LEAD LAB TECH. ON 5/26/07 EMPLOYEE WENT TO THE ER. DIAGNOSIS FRACTURE OF THE ANKLE.