The initial public offering (IPO) of wearable fitness device maker Fitbit Inc. (NYSE: FIT) went out the door Thursday morning with an opening price of $31.34 per share. Shares had priced at $20 on Wednesday, giving the stock an opening lift of 52%.

Fitbit boosted its share offering from a planned total of 29.9 million to 36.6 million and priced the shares on Wednesday at $20, above the upwardly revised range of $17 to $19. The prior range had been $14 to $16.

The company sold about 22.4 million shares and existing shareholders sold about 12.1 million in the IPO. Fitbit has a dual-share structure, but all the shares sold Thursday are Class A common shares.

Class B common stock is identical to the Class A shares except in voting and conversion rights. Class B shares are entitled to 10 votes per share, and they may be exchanged one-for-one for Class A shares at any time. Class B shares hold about 98% of the voting power following the IPO, and Fitbit’s directors, executive officers and significant stockholders hold about 52.8% of the Class B shares.

Lead underwriters for the offering are Morgan Stanley, Deutsche Bank and Bank of America Merrill Lynch. Co-managers include Barclays, SunTrust Robinson Humphrey, Piper Jaffray, Raymond James, Stifel and William Blair. The underwriters have received an overallotment option from the selling shareholders for an additional 5.18 million shares.

Fitbit said in its IPO filing documents that it had sold 20.8 million of its wearable devices since 2009. More than half of that total was sold in 2014, and the company sold 3.9 million devices in the first quarter of this year.

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The high valuation (the company raised gross proceeds of more than $700 million at a market value of around $3.7 billion) and the opening bump to the share price may indicate that investors see Fitbit’s devices as a legitimate alternative to the Apple Watch from Apple Inc. (NASDAQ: AAPL). A recent estimate by online order tracker Slice Intelligence puts the number of Apple Watches sold in the United States at about 2.8 million.

That’s a big number, but more than half of those watches — 1.5 million — were sold on the first day of availability. And more than half of that 1.5 million — an estimated 800,000 — were sold in the first hour. Since then, sales have been running at about 30,000 a day, or an annual rate of about 11 million.

Another research firm, KGI, has cut its forecast for Apple Watch sales in the third quarter to 5 million to 6 million units and annual sales have been cut to below 15 million. The general consensus for Apple Watch sales had been between 20 million and 30 million units shipped in the first year.

The wearable device market is set to grow from 45.7 million in 2014 to 126.1 million by 2019, according to IDC. Of the total market, nearly 90% will be wrist-worn wearables in 2015, or about 41 million devices. When IDC published this forecast in March, the analyst noted:

The Apple Watch raises the profile of wearables in general and there are many vendors and devices that are eager to share the spotlight. Basic wearables, meanwhile, will not disappear. In fact, we anticipate continued growth here as many segments of the market seek out simple, single-use wearable devices.

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The most expensive Fitbit device, the Surge, has a list price of $249.95, a full $100 lower than the least expensive Apple Watch. The cheapest Fitbit device, the Zip, retails for $59.95.

It is way too early to predict the impact that Apple Watch will have on Fitbit devices, but investors Thursday are betting either than Apple will not hurt Fitbit’s business or that the market is going to be even bigger than IDC thinks. Either way, Fitbit is looking fit and healthy.

After popping to a high of $31.90, Fitbit shares traded at about $29.90, up nearly 50%, after touching a low of $29.50.

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