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Wearables maker Fitbit Inc. (NYSE: FIT) is losing its chief financial officer to an unspecified autonomous-vehicle startup on June 15. Bill Zerella, who has held the post since 2014, will be replaced as CFO by Ron Kisling, currently the company’s chief accounting officer.

Zerella told MarketWatch that the move is a “personal decision [that gives him] the opportunity to take an earlier-stage company toward rapid-stage growth in the future.” He joined Fitbit about a year before the company’s June 2015 initial public offering.

Fitbit CEO James Park said:

Over the past four years, Bill’s passion and dedication to growing early stage companies were key enablers of Fitbit’s transformation from a startup to the leading global brand it is today. We thank him for his countless contributions, including leading our successful IPO in 2015, guiding our worldwide expansion, and laying the foundation for our future success, and wish him all the best in his next endeavor.

That’s not to say that Fitbit hasn’t struggled since its IPO. The stock quickly jumped to nearly $48 a share before tumbling to an all-time low below $5 earlier this year. In the past 12 months, though, the stock has added about 17%, a larger gain than the S&P 500 for the same period.

The company began selling its Versa device a month ago and said on Tuesday that it has sold 1 million of the health and fitness-focused smartwatch since then. It is the company’s fastest-selling device ever.

New CFO Kisling has been with Fitbit for nearly four years and was CFO of publicly traded semiconductor firm Nanometrics before coming to Fitbit.

Fitbit stock took a hit in early trading Wednesday, down more than 4.5% at $5.96. The stock’s 52-week range is $4.51 to $7.32 and the 12-month consensus price target is $6.06.