Four small river systems in Hesse, Germany, were investigated with respect to seasonal and spatial concentrations of the herbicide terbutryn [2-(t-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine]. Despite introduction of a ban on its use as a herbicide in July 2003, terbutryn was still present in the rivers during the whole sampling period from September 2003 to September 2006, and there was no trend of decreasing concentration during this time. In the Weschnitz and Modau river systems the mean terbutryn concentration exceeded the German drinking water ordinance threshold value for single biocides. Maximum concentrations of up to 5.6 microg l(-1) were determined in the Weschnitz River. Higher terbutryn concentrations in summer are suggested to originate from agricultural sources, as well as from sediment redissolution. Effluents of two sewage treatment plants had high terbutryn concentrations, indicating that terbutryn enters the rivers from this source. Sources other than agriculture must explain terbutryn occurrence in the rivers during winter, when farm pesticide application typically ceases. The potential for mobilization of terbutryn from sediments and leaching from soils are discussed.