Groundwater in the province of "Zuid-Holland" was investigated for residues of pesticides. Aim of this investigation was to get more information about the threat of groundwater quality as a consequence of application of pesticides, in order to take possible measures for protection in higher concentrations than the EC-guideline for drinking water of 0.1 mug/l for one compound or 0.5 mug/l for the total of several residues of pesticides. Five different compounds were found. The decomposition product ETU (ethylenthiourea) which is formed after application of several fungicides was found in four samples in concentration range of 0,2-14,4 mug/l. At a depth of 7-9 m 7,2 mug/l was found. In five samples 1,2-dichloropropane was found in concentrations of 1.5 to 22.7 mug/l, while at a depth of 8-9 m 7.8 mug/l was found. Furthermore three compounds were found in one sample: 1,3-dichloropropane, chloroallylalcohol and aldicarbsulphon. The compound 1,2-dichloropropane, which is a consequence of soil desinfection, was also found in groundwater below glass houses (4x), potato cultured land (3x) and in bankinfiltration (1x). Carbendazim (0.1 mug/l) was found in groundwater below fruit cultured land.