A composed free kick from Michael Bostwick has Peterborough United ahead at the break against managerless Barnsley.
Dwight Gayle and Lee Tomlin looked dangerous for Posh from the outset as they went in search of a fourth win in five matches.
Bostwick broke the deadlock on 30 minutes with a neat set piece on the edge of the area.
Joe Newell came close to doubling the visitors' advantage with a low strike before Barnsley had their best chance of the half in stoppage time.
A free kick from Tom Kennedy took a nasty deflection and needed a strong hand from Robert Olejnik to keep the Tykes goalless at the break.