Professional employer organization (PEO) revenue at Automatic Data Processing Inc. (NASD: ADP) rose 15.5 percent to $413.8 million in the company’s fiscal second quarter ended Dec. 31, 2011, from revenue of $358.2 million in the year-ago quarter.

The average number of paid PEO worksite employees rose 13.6 percent in the second quarter to approximately 251,000 from 221,000 in the second quarter of the previous year.

ADP reports PEO revenue excluding direct pass-through costs such as payroll wages and payroll taxes. Those direct costs totaled $4.81 billion in the second quarter of this year and $4.23 billion in the second quarter of the previous year.

PEO services represent just one of Roseland, N.J.-based ADP’s business lines, which include non-staffing services. Total second-quarter revenue at the firm was $2.58 billion, up 7.4 percent on a year-over-year basis.

ADP’s second-quarter net income rose 20.9 percent to $375.0 million.