ABSTRACTGround‐water samples from two heavily fertilized sites in Suffolk County, New York, were collected through the 1978 growing season and analyzed for nitrate‐N concentrations and nitrogen‐isotope ratios. Six wells were at a potato farm; six were on a golf course. The purpose of this study was to determine whether the15N/14N ratios (δ15N values) of fertilizer are increased during transit from land surface to ground water to an extent which would preclude use of this ratio to distinguish agricultural from animal sources of nitrate in ground water.Ground water at both sites contained a greater proportion of15N than the fertilizers being applied. At the potato farm, the average δ15N value of the fertilizers was 0.2%0; the average δ15N value of the ground‐water nitrate was 6.2 %0. At the golf course, the average δ15N value of the fertilizers was ‐5.9%0, and that of ground‐water nitrate was 6.5%0. The higher δ15N values of ground‐water nitrate are probably caused by isotopic fractionation during the volatile loss of ammonia from nitrogen applied in reduced forms (NH+4and organic‐N).The δ15N values of most ground‐water samples from both areas were less than 10%0, the upper limit of the range characteristic of agricultural sources of nitrate; these sources include both fertilizer nitrate and nitrate derived from increased mineralization of soil nitrogen through cultivation. Previous studies have shown that the S15N values of nitrate derived from human or animal waste generally exceed 10%0. The nitrogen‐isotope ratios of fertilizer‐derived nitrate were not altered to an extent that would make them indistinguishable from animal‐waste‐d