Danny Haynes' first goals for Barnsley proved enough to sink south Yorkshire rivals Doncaster 2-0.
Haynes, who joined the Tykes from Bristol City last week, headed home a right-wing cross from Bobby Hassell nine minutes into the second half to give his side the lead.
An unusually subdued Doncaster struggled to respond and fell further behind 13 minutes later when Haynes cut in from the right and fired in a low left-footed shot that squirmed under the body of Neil Sullivan.
The visitors held on comfortably thereafter to record their first away win of 2011 and leapfrog their opponents in the npower Championship table.
Doncaster have now lost four consecutive games in all competitions and their misery was compounded by injuries to Mark Wilson, James Chambers and Matt Kilgallon.