Talon Thornton, director of Springfield's Henson Robinson Zoo since 1995, has resigned.
Thornton submitted his resignation Tuesday, but no immediate explanation was given. He plans to stay on at the zoo until a replacement is chosen.
The zoo's accreditation was put on hold last year, and the park district recently implemented a master plan to oversee improvements at the zoo.
Neither Thornton nor acting park district director Derek Harms returned calls seeking comment.