Professional employer organization revenue at Automatic Data Processing Inc. (NYSE: ADP) rose 18.4% to $277.1 million in its fiscal first quarter ended Sept. 30.

The Roseland NJ-based company said the average number of worksite employees rose by 15.5% to approximately 190,000 compared with the same period in the previous year.

ADP lowered its expected increase in PEO revenue for fiscal year 2009 to between 14% and 16% growth from its previous estimate of 16% to 17%. It also said new business sales would decline by about 10%. President and CEO Gary Butler said turmoil in the financial sector has weakened the selling environment for PEO services.

ADP also provides payroll processing services to employers and services to auto dealers. Total first-quarter revenue at ADP rose 9.5% to $2.18 billion. Net income, however, fell 6.9% to $276.9 million.