Shares of Dollar General (DG) plunged 6.5% on Friday after long-time chief executive Rick Dreiling unveiled plans to retire no later than May 2015.

Dreiling, 60, was named CEO in January 2008 and became chairman of the board that December. During his six years at the helm, Dollar General’s shares climbed more than 80% to $17.5 billion in 2013. Store count has increased by 38% to more than 11,000 stores.

“Rick has driven significant shareholder value and led the company’s improvement on essentially every key operating metric,” lead director Mike Calbert said in a statement. “Rick has proven that he is clearly one of retail's leading CEOs.”

His retirement date is slated for May 30, 2015, though that could move depending on when a successor is named. The board said it is conducting both an internal and external search, and Dreiling has agreed to serve as chairman during the transition period.

While the company said it is confident it is well-positioned to handle this transition, investors were seemingly spooked. Its shares slumped 7.2% to $57.22 in recent trade.

Follow Jennifer Booton on Twitter at @Jbooton