Monitoring of the contamination of water bodies by organic contaminants such as pesticides and the determination of reliable estimates of concentrations is a challenging issue, in particular for implementation of the Water Framework Directive. Integrative (or passive) sampling has recently been developped as an alternative to grab or average automated sampling, in order to obtain at lower cost, more realistic estimates of the average concentrations of contaminants in surface waters. These devices allow the passive concentration of chemicals from large volumes of water, resulting in ultratrace level detection and smoothed integrative sampling. If well calibrated, they allow the determination of time-weighted average (TWA) concentrations of the target compounds. Stir Bar Sorptive Extraction (SBSE) is a solvent free sample preparation technique dedicated to the analysis of moderately hydrophobic compounds in aqueous samples. It is composed of a magnet enclosed in a glass tube coated with a thick film of polydimethysiloxane (PDMS or silicone). The aim of this study was to develop and validate the in situ application of SBSE as a passive sampling technique for the monitoring of 25 pesticides in two small rivers of a French agricultural watershed. Stir bars were deployed for periods of one week at both sites. TWA concentrations could be determined, after laboratory calibration of SBSE. Then, results were compared to those obtained by means of grab or automated sampling. The use of SBSE applied in situ allowed to measure with good repeatability more hydrophobic pesticides and at lower detection levels than the classical sampling techniques.