Measured NO3- concentrations in the Jersey bedrock aquifer exceeded 50 mgl-1 in 67% of groundwaters sampled in June 1995. The δ15N values of this dissolved NO3- showed a total range in composition from +3.6‰ to +18.4‰ and exhibited a general, island-wide increase from +4‰ in the centre of the island to +9‰ towards coastal districts. The majority of the island is intensively cultivated and most of the sites sampled have δ15N values of less than +7‰ (mean = +7.7‰, n = 40) indicative of an agricultural source for NO3-. There is no apparent evidence for point-source contamination from septic tanks. The progressive seaward increase in δ15N is attributed to an increasing volume of deeper, denitrified groundwater mixing with shallow groundwater. Values of δ15N greater than +10‰ are restricted to those coastal and valley locations that abstract, due to their low topographic elevation, the greatest proportion of deeper, denitrified groundwater. An alternative explanation is that sewer leakage enhances the observed δ15N values in southern urban areas, although this is considered unlikely to be occurring at present.