A header from substitute Bobby Hassell hands Barnsley the lead against Watford after 45 minutes.
Barnsley have defended doggedly and taken advantage at the other end to lead promotion chasers Watford at the interval.
Gianfranco Zola's side are the top scorers away from home in the Championship and made a bright start when Ikechi Anya caused panic in the Barnsley backline.
Rory Delap was forced off injured after just five minutes for the Tykes and replaced by Bobby Hassell.
Matej Vydra fired well over before Lloyd Doyley stopped Jason Scotland from pouncing at the other end.
The Tykes took the lead 10 minutes before the break when Hassell climbed highest to meet a precise delivery from Jacob Mellis at the far post.
Watford were unable to respond in a frantic final passage of play as Barnsley held firm to take a narrow advantage into the second half.
Team News: Two changes for Bristol, six for Watford >