** Shares of MGM China Holdings 2282.HK drop as much as 12.1% to HK$15.23, their lowest since July 28 and on course to snap three consecutive session of gains

** Stock posts biggest intraday pct decline since April 7, and is biggest pct loser among the six key HK-listed Macau gaming groups

** MGM China said its first-half net profit fell 11.2% y/y to HK$2.38 bln ($303.19 mln) but casino revenue was up 3.1% y/y

** Says its overall gaming market share was 16.2% for the six-month period ended in June, down from 16.5% the same period a year ago

** Shares of rivals also fall with Sands China 1928.HK, Galaxy 0027.HK, SJM 0880.HK, Melco International 0200.HK and Wynn Macau 1128.HK down between 0.6% and 3.8%

** Hang Seng Commerce & Industry Index .HSNC and benchmark Hang Seng Index .HSI both slip 0.5%

** YTD, MGM China shares up 65.8%

($1 = 7.8498 Hong Kong dollars)

(Reporting by Donny Kwok)

((donny.kwok@thomsonreuters.com))