Nasdaq OMX handled nearly a third of all U.S. options trading in December, giving the exchange operator its biggest slice yet of a fast-growing market.

Nasdaq PHLX and its sister platform NOM accounted for 32.7 percent of all stock- and stock-index-options trades in December, figures from the Options Clearing Corp showed. Overall, trading at the nine U.S. options exchanges rose about 16 percent to a daily average of 15.5 million contracts.

But much of the trading on Nasdaq’s PHLX venue comes from a specialized trading strategy linked to bets on stock dividends that rivals say distort the overall market.

The International Securities Exchange, owned by Deutsche Boerse’s futures exchange Eurex, estimated that over 40 percent of trading at PHLX was from such trades, “which inflate and distort trading volume and market share,” ISE said in a statement.

Based on OCC figures, which include the trades in question, ISE ranked No. 4 in U.S. options trading, behind Nasdaq, CBOE Holdings and NYSE Euronext, in that order.

But both CBOE and ISE said their true share of the market is much higher if the dividend trades are excluded, and both argue that they should be. PHLX caps fees on the trades, so they are much less profitable than other trading volume.

Trading at CBOE in December sank 6.6 percent from a year earlier; at ISE, trading fell 6.4 percent. Trading at PHLX jumped 55 percent, OCC figures show.