The Chicago Board Options Exchange (NASDAQ: CBOE) had the top market share of options trading volume in April, though its hold slipped a little, according to trading statistics compiled by the Options Clearing Corp.

The CBOE and its C2 division held a combined market share of 27.74% of total options volume, beating out seven other options exchanges. Finishing second was the NASDAQ OMX Group (NASDAQ: NDAQ) with its NASDAQ OMX PHLX and NASDAQ OMX divisions combining for a share of 25.70%.

Year over year, the CBOE share dropped from nearly 31% in April 2010, while the NASDAQ divisions rose from 23.91% a year ago.The intriguing story this April was the rise of the NYSE AMEX exchange, garnering a 14.18% market share, up from 10.65% in the same period a year ago. The NYSE AMEX is a division of NYSE Euronext (NYSE: NYX)  And the BATS Options Exchange lodged a 3.41% share, up from 0.20% in April 2010.

The OCC breaks out options volume in total trading and its major subset — equity and exchange-traded funds (ETFs). Trading volume on the equity side typically accounts for about 90% of all option trading while index options trading accounts for the rest.

The two NASDAQ exchanges led equity/etf option trading in April, taking a 27.34% market share, followed by CBOE with 22.19%, and the International Securities Exchange (ISE) with 18.32%.

CBOE continues to lead in the options marketplace because it dominates index options trading, typically holding a market share of more than 95% with products like the S&P Index Options (CBOE: SPX) and the CBOE Volatility Index (CBOE: VIX). The VIX is also widely known as the “Fear Gauge” because some contend it is an indicator of a market decline.

The nine U.S. options exchanges are BATS, the Boston Options Exchange (BOX), CBOE and its C2 division, ISE, NASDAQ OMX and NASDAQ OMX PHLX, NYSE AMEX and NYSE Arca.