Oct. 11 (Bloomberg) -- The ranks of South Africa's black middle class swelled 15 percent over the past year, even as rising interest rates ate into consumers' disposable income, a new study has found.

About 3 million black South Africans are now considered middle class, earning an average of 7,000 rand ($747) a month, according to the study carried out by the University of Cape Town's Unilever Institute of Strategic Marketing and TNS Research Surveys.

``Despite dire predictions that this sector of society is facing a financial melt-down due to debt pressure, the opposite is true,'' John Simpson, the institute's director, said in an e- mailed statement today. ``The most astounding evidence of their financial resilience can be seen in a 39 percent increase in spending power, from 180 billion rand in 2007 to 250 billion rand this year.''

Under apartheid rule black South Africans, who account for about 80 percent of a population of 48.5 million people, were largely excluded from the mainstream economy. Since all-race elections in 1994, the government has sought to address the situation by forcing companies to sell stakes to black investors and increasing access to education.

The growth rate of the middle class, whose ranks includes professionals such as teachers and tertiary education students, was half the 30 percent increase experienced between 2006 and 2007.

``We are working off a bigger base, which means that growth will inevitably slow in percentage terms,'' said TNS research manager Rudo Maponga. ``Despite the downturn, their spending power has continued to grow, which suggests that this segment is also becoming wealthier.''

To contact the reporter on this story: Mike Cohen in Cape Town at mcohen21@bloomberg.net

Last Updated: October 11, 2008 06:00 EDT