This investigation is the first regionally focused study of the presence of natural radioactivity in water from the Vincentown and Wenonah-Mount Laurel aquifers, Englishtown aquifer system, and the Hornerstown and Red Bank Sands. Geologic materials composing the Vincentown and Wenonah-Mount Laurel aquifers and the Hornerstown and Red Bank Sands previously have been reported to contain radioactive (uranium-enriched) phosphatic strata, which is common in deposits from some moderate-depth coastal marine environments. The decay of uranium and thorium gives rise to natural radioactivity and numerous radioactive progeny, including isotopes of radium. Naturally occurring radioactive isotopes, especially those of radium, are of concern because radium is a known human carcinogen and ingestion (especially in water used for drinking) can present appreciable health risks. A regional network in southwestern and south-central New Jersey of 39 wells completed in the Vincentown and Wenonah-Mount Laurel aquifers, the Englishtown aquifer system, and the Hornerstown and Red Bank Sands was sampled for determination of gross alpha-particle activity; concentrations of radium radionuclides, major ions, and selected trace elements; and physical properties. Concentrations of radium-224, radium-226, and radium-228 were determined for water from 28 of the 39 wells, whereas gross alpha-particle activity was determined for all 39. The alpha spectroscopic technique was used to determine concentrations of radium-224, which ranged from less than 0.5 to 2.7 pCi/L with a median concentration of less than 0.5pCi/L, and of radium-226, which ranged from less than 0.5 to 3.2 pCi/L with a median concentration of less than 0.5 pCi/L. The beta-counting technique was used to determine concentrations of radium-228. The concentration of radium-228 ranged from less than 0.5 to 4.3 pCi/L with a median of less than 0.5. Radium-228, when quantifiable, had the greatest concentration of the three radium radioisotopes in 9 of the 12 samples (75 percent). The concentration of radium-224 exceeded that of radium-226 in five of the six (83 percent) samples when both were quantifiable. The radium concentration distribution differed by aquifer, with the highest Ra-228 concentrations present in the Englishtown aquifer system and the highest Ra-226 concentrations present in the Wenonah-Mount Laurel aquifer. Radium-224 generally contributed a considerable amount of gross alpha-particle activity to water produced from all the sampled aquifers, but was not the dominant radionuclide as it is in water from the Kirkwood-Cohansey aquifer system, nor were concentrations greater than 1 pCi/L of radium-224 widespread. Gross alpha-particle activity was found to exceed the U.S Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) of 15 pCi/L in one sample (16 pCi/L) from the Vincentown aquifer. A greater part of the gross alpha-particle activity in water from the Wenonah-Mount Laurel aquifer resulted from the decay of Ra-226 than did the gross alpha-particle activity in the other sampled aquifers; this relation is consistent with the concentration distribution of the Ra-226 itself. Concentrations of radium-224 correlate strongly with those of both radium-226 and radium-228 (Spearman correlation coefficients, r, +0.86 and +0.66, respectively). The greatest concentrations of radium-224, radium-226, and radium-228 were present in the most acidic ground water. All radium-224, radium-226, and radium-228 concentrations greater than 2.5 pCi/L were present in ground-water samples with a pH less than 5.0. The presence of combined radium-226 and radium-228 concentrations greater than 5 pCi/L in samples from the Vincentown and Wenonah-Mount Laurel aquifers and the Englishtown aquifer system was not nearly as common as in samples from the Kirkwood-Cohansey aquifer system, likely because of the slightly higher pH of water from these aquifers relative to that of Kirkwood-Cohansey aqu