The Institute for Supply Management says its service-sector index fell to 51.5 last month from 54.3 in July. Economists had expected a higher reading of 53.5.
Levels above 50 signal growth.
The index shows service companies have been expanding every month this year, but not as fast as the much smaller manufacturing sector. Companies that supply services, which range from hospitals to shops and banks, depend more on spending by consumers in the U.S.
The survey's gauge of future business, new orders, slowed to the weakest pace this year.