Washington Wild Things manager Bart Zeller, 72, resigned Sunday citing "health reasons and concerns."
Zeller, who stepped down before a home game against the Florence Freedom, leaves with the Wild Things at 32-19 and in first place in East Division of the independent Frontier League.
No interim manager has been named.
Zeller did not manage in a 5-1 victory Sunday against Florence, the Wild Things' final game before the Frontier League All-Star break.
Zeller gave Wild Things owner Stu Williams a resignation letter, which did not go into detail about his health issues.
"I respect his privacy, and we're leaving it alone and moving on," Williams said.
Williams will discuss the manager situation during the All-Star break with hitting coach Bob Didier, pitching coach Kevin Gryboski and bench coach Bob Bozzuto. Because of the All-Star break, the Wild Things do not play again until Friday at home against the River City Rascals.
"I trust these fellow coaches will help me make a good decision for the team," Williams said.
Zeller had been slotted to manage the East Division in the All-Star Game Wednesday in Sauget, Ill. The league announced Sunday that he would be replaced by Evansville Otters manager Andy McCauley.
This season was Zeller's second as manager of the Wild Things. The team finished 41-55 in 2013 before their resurgence this season. He also spent two seasons managing the Joliet Slammers in 2011 and 2012, winning the Frontier League championship in 2011.
Heading into the All-Star break, the Wild Things hold a one-game lead in the East Division. Six teams make the Frontier League playoffs -- the East and West division leaders and the next four best records. Washington has not had a winning season since 2007.
"I talked to the ballclub about it and management," Zeller said Monday. "It was purely health reasons, something I had no knowledge of until recently."
Zeller declined to comment further on his decision.
Williams said the team responded to the unexpected news like professionals. Didier, Gryboski and Bozzuto handled coaching duties Sunday.
Even though Zeller was an integral part of this season's turnaround, Williams is confident the team will continue to move in a positive direction.
"We worked hard putting this team together during the offseason, but it was very much a team effort," Williams said. "I can't say enough about the team on the field. They've made plays and come back in situations like I've never seen a Wild Things team in 13 years do."
Sean Hammond: firstname.lastname@example.org, 412-263-1466 and Twitter @sean_hammond.