(Adds details, comments form analyst and company spokesperson, updates share movement)

By Sweta Singh

BANGALORE, July 24 (Reuters) - Derivatives trader Interactive Brokers Group Inc (IBKR.O: Quote, Profile, Research) reported second-quarter profit below market estimates as interest income fell, sending its shares down more than 17 percent. The Greenwich, Connecticut-based company, which went public in May 2007, reported quarterly net income of $18.3 million, or 44 cents a share, compared with a pro-forma income of $11.6 million, or 28 cents a share, in the year-ago period.

Total net revenue increased 34 percent to $395.4 million.

Three analysts on average had expected the company to earn 50 cents a share, before special items, according to Reuters Estimates. Two analysts on average had expected revenue of $427.4 million.

Earnings were not as good as the first quarter, when the company made a lot of money in the market making side, senior VP of product development at Interactive Brokers, Steven Sanders, told Reuters.

"Given all the volume and everything, they (analysts) were probably expecting a little more on the market making side."

The company's market-making segment had net revenue of $271.7 million during the quarter.

"Market making revenue was less than expected," said Sandler O'Neill & Partners analyst Richard Repetto, who had an estimate of $308.0 million for the segment.