In 1989 several groundwater wells below agricultural land, used for potato-, maize- and flowerbulb culture, as well as uncropped land has been sampled and analysed for pesticides. Below vulnerable soil types as well as less vulnerable soil types which are representative for an average Dutch sandy soil several pesticide residues have been found. Since the start of this sampling program in 1988, 25 differend organic compounds are residues of 15 pesticides. Nine of them are allowed in groundwater protection areas: atrazine, amitrol, diuron, dichlobenil, dinoseb, ethoprophos, maneb/zineb, metamitron and metam-sodium, while the other six are not allowed in groundwater protection areas: aldicarb, 1,3-dichloropropene, bentazon, dimethoate, metolachlor and simazine. The highest concentrations were found for transformation products of pesticides and admixtures. In particular transformation products are a threat for the quality of groundwater and drinking water supply.