Field work in the island of St. Croix, V. I., was carried on from December 1938 to April 1939 in connection with a test-drilling program for water sup- plies. The island is 21 miles long and has a maximum width of 6 miles. Its western part consists of a range of mountains flanked on the south by a rolling plain; its narrower eastern part is entirely mountainous. There are only a few small streams. The rolling and fiat lands are cultivated or are in grass, and the mountainous areas are either wooded or in grass. The average rain- fall of the island is 46.34 inches, but severe droughts and periods of excess precipitation are not uncommon. The island is made up of rocks of Upper Cretaceous age, mostly volcanic tufts and limestones known as the Mount Eagle volcanics; diorite intruded into the cretaceous rocks; and Oligocene to Miocene blue clays and yellow marls (the Jealousy formation and Kingshill marl, respectively). Alluvium is widely distributed. The Mount Eagle rocks were strongly folded in early Tertiary time and the Kingshill strata gently folded in post Lower-Miocene time along an east-northeast axis. Three early Tertiary cycles of erosion are recognized. After the folding of the Kingshill marl, streams followed the strike of the folded rocks in a westerly direction, but they gradually assumed southward courses across the marl plain and as a result a western area of old-age topography, a central area of late-mature topography, and an eastern area of early-mature topography have been created. Submerged reefs and emergent reefs and beaches indicate several fairly recent stands of the sea. Water for human consumption is obtained by collecting rain water in cis- terns, but water for other purposes is almost entirely supplied by wells which are generally less than 100 feet deep. Many dug wells are used, but in recent years drilled wells have been constructed. Most of them are discharged by wind-powered pumps of small capacity. Wells are developed in all the rocks mentioned (except coral reef), but the best yields are obtained from the alluvium. A maximum yield of 80 gallons a minute was obtained from a gravel-packed well in the alluvial valley at Fair Plain. Further exploration of the alluvium is recommended. The weathered diorite also appears to be a fairly good water-bearing formation. Test drilling showed that deep water- bearing formations should probably not be expected beneath the Tertiary rocks. Most of the ground waters of St. Croix contain a moderately high mineral content owing to the solution of rock-forming minerals and the deposition of alkali and salt spray in the soil. Only a few wells are contaminated by sea water. The low hardness of some highly mineralized waters is believe due to base exchange. The most highly mineralized waters are found in the alluvium in areas with alkali soil and in some places in the Tertiary limestones where presumably soluble salts were deposited in those strata. The least mineralized waters are found in shallow wells in the alluvial near the foot of the mountains and in the areas of dioritic rock. Many well waters in Croix, if properly protected from contamination might be entirely suited to human consumption. Although many waters are hard, they are used for domestic purposes. Most waters, even those high in chloride, are reported to be excellent for cattle consumption. Most ground waters in St. Croix cannot be used for boiler feed without treatment but are used for o*her purposes in the manufacture of sugar and rum. A brief discussion of the results of test drilling by the National Park Service in 1940-41 is also given.