MasterCard Inc. will replace General Motors Corp. in Standard & Poor's S&P 100 index of stocks, Standard & Poor's announced yesterday. The change takes effect after the close of trading on July 17. On the S&P 500 index, MasterCard replaced Ace Ltd. Ace, a property and casualty insurance and reinsurance company, is changing its place of incorporation to Switzerland, which makes it ineligible for inclusion in S&P U.S. indices, Standard & Poor's stated in a news release. Though GM was dropped from the top 100 index, it will continue to be a member of the S&P 500. S&P did not say in its statement why it is dropping GM from the S&P 100. But on June 20, Standard & Poor's Ratings Services placed the corporate credit ratings of GM, Ford Motor Co. and Chrysler LLC on its CreditWatch list with negative implications. The ratings service attributed the warning to flagging demand for large consumer vehicles because of rising fuel prices. GM stock traded on the New York Stock Exchange at $9.84 per share in late-afternoon trading today, down from $36.82 per share a year ago. MasterCard stock, meanwhile, traded at $257.88 per share this afternoon, up from $156.35 a year ago and six times its share price when MasterCard went public in May 2006. Companies must trade stock for at least one year before they can join Standard & Poor's indices, which means Visa will not be eligible for inclusion until at least February 2009, a year after the card brand's initial public offering.