WASHINGTON (AP) — U.S. service companies grew in September at the fastest pace in six months, helped by a sharp increase in customer demand.

The Institute for Supply Management, a trade group of purchasing managers, said Wednesday that its index of non-manufacturing activity rose to 55.1, up from 53.7 in August. Any reading above 50 indicates expansion.

The report measures growth at businesses that employ roughly 90 percent of the U.S. work force, from retail and construction companies to health care and financial services firms. The service sector has grown for 33 straight months.