An investigation of the geology and ground-water conditions in a part of southwestern Long Island, including the construction of 8 deep and 5 shallow test and observation wells, has been completed by the U. S. Geological Survey in cooperation with the Nassau County Dept. of Public Works and the New York State Water Power and Control Com- mission. In this area are 4 main water-bearing units of Late Cretaceous and Pleistocene age. Ground water having a higher than normal salt content is encountered in some wells that penetrate the upper 3 units. Although the presence of this water constitutes a potential threat, existing fresh-water supplies apparently are in little immediate danger under present conditions. The "salty" ground water has chloride concentrations ranging from about 40 to 18,000 p. p. m. (parts per million), generally more than about 2,000 p. p. m. It occurs in several bodies, seaward of and beneath the fresh water in the respective aquifers, and is in contact with the fresh water through zones of diffusion. The largest salt-water body, confined under artesian pressure, is in the Jameco gravel and the Magothy(?) formation beneath extreme southwestern Nassau County and adjoining southeastern Queens County. This body is thought to have occupied a substantial part of its present extent since at least 1895. It probably has been and is now encroaching, very slowly, because of increased pumping from the fresh-water bodies and possibly as a result of a postglacial rise of sea level. The rate of encroachment under present conditions probably is less than 100 ft. per year. Application of the Ghyben-Herzberg principle to compute depths to the contact between salty and fresh ground water was found to give erroneous results. More accurate depths and a clearer understanding of the relation between heads in adjoining bodies of fresh and salty ground water are obtained by use of a formula adapted from one given by M. King Hubbert.