* Indexes up: Dow 0.9 pct, S&P 0.8 pct, Nasdaq 0.9 pct
By Ryan Vlastelica
NEW YORK, Sept 24 (Reuters) - Cyber-Ark Software Ltd was the Nasdaq's biggest gainer on Wednesday, with the stock soaring on massive volume in its trading debut.
The stock opened at $25, well above its initial public offering price of $16, and added to those gains in afternoon trading. It last traded up 67 percent at $26.86, on volume of 16.43 million shares, making the Israel-based cybersecurity software company one of the Nasdaq's most active names.
Cyber-Ark's strong debut follows Alibaba Group Holding Ltd's from last week. That IPO ranks as the world's biggest, and shares rose 38 percent in their trading debut on Friday.
Index snapshot at 15:02 EDT:
* S&P 500 was rising 15.48 points, or 0.78 percent.
* Nasdaq Comp was gaining 41.90 points, or 0.93 percent.
* Dow industrials was adding 158.99 points, or 0.93 percent.
* Russell 2000 was adding 9.44 points, or 0.84 percent.
* S&P MidCap was adding 6.7 points, or 0.48 percent.
* S&P SmallCap was gaining 5.38 points, or 0.84 percent.
Editing by Meredith Mazzilli