The Institute for Supply Management reported Friday that its service index dipped to 58.6 last month, down from a reading of 59.6 in August which had been the strongest level recorded since the measure was introduced in January 2008.
Hiring at service firms, where most Americans work, rose for a seventh straight month.
The service sector index covers about 80 per cent of the private sector economy. The index tracks new orders, business activity, employment and supplier delivery delays.
The hiring gains were led by increases in construction and retail trade. Among the comments was one which said that the "previous conservative staffing model is relaxing due to increased demand for services."
A total of 12 service industries reported growth in September. Five reported contraction.
On Wednesday, the purchasing managers group reported that its index of manufacturing activity slipped slightly in September to a reading of 56.6, down from 59 in August.
For both indexes, any reading above 50 signals growth.