As part of the National Water-Quality Assessment (NAWQA) program, microbiological data were collected from wells in 22 NAWQA study units during 1993-2004. The wells constituted the sampling networks for three major NAWQA efforts--the major aquifer study, the land-use study, and source-water quality assessments of ground water used for public supplies. Sixteen principal aquifers were represented by these well networks. Samples of untreated ground water were analyzed for concentrations of fecal-indicator bacteria, which included the total-coliform bacteria, fecal-coliform bacteria, and Escherichia coli, and for the presence of somatic and male-specific coliphage viruses. Analyses of the samples showed that coliform bacteria occur relatively frequently-nearly 30 percent of all wells tested positive-and that domestic wells commonly are contaminated by total coliform bacteria, with 33 percent of these wells testing positive. Coliphage viruses were present in 10 percent or fewer of the wells sampled in the Central Columbia Plateau-Yakima, Georgia-Florida, San Joaquin, and Trinity study units, which represent the Columbia Plateau, Floridan, Central Valley, and Coastal Lowlands principal aquifers, respectively. The frequency of detections and concentrations of total coliform bacteria generally were higher in samples from domestic wells than in samples from public-supply wells; in fractured or porous rock materials (carbonate rocks) than in unconsolidated materials (mixtures of sand, gravel, clay); and in principal aquifers with median depths of sampled wells ranging from 100 to 200 feet than in principal aquifers with median depths of sampled wells less than 100 feet or greater than 200 feet. The waters most affected by the presence of coliform bacteria were those in the Valley and Ridge, the Floridan, and the Piedmont and Blue Ridge aquifers, where more than 50 percent of the study wells tested positive for these bacteria. The numbers of wells with detections of coliform bacteria were significantly lower for the Glacial Deposits, Stream and River Valley, Columbia Plateau, Basin and Range, High Plains, Southeastern Coastal Plain, and Coastal Lowlands aquifers. Of the 16 principal aquifers sampled, wells in the Valley and Ridge had the highest overall concentrations of total coliforms, with a median of 2 colony-forming units per 100 milliliters. Elevated concentrations of coliform bacteria (greater than 300 colony-forming units per 100 milliliters) also were reported for wells completed in the Mississippian-Pennsylvanian aquifer and the Ordovician aquifer in lower Tennessee. More than 50 percent of wells completed in carbonate or crystalline rocks tested positive for coliform bacteria. The Floridan, Piedmont and Blue Ridge, Ordovician (lower Tennessee), and Valley and Ridge aquifers are composed of these types of rocks. The lowest detection frequencies (less than 5 percent) were for wells in the Basin and Range and the Snake River aquifers. The materials that constitute these aquifers primarily are unconsolidated sand, gravel, and clay, or basalt with interbeds of sand, gravel, or clay. No strong correlations between concentrations of total coliforms and well depths were identified, nor were there any correlations between total coliform concentrations and selected chemical constituents in the waters. Other factors, such as geohydrologic characteristics, proximity of contaminating sources, interactions with surface water, or well-construction features (including the age of the well), likely control the presence and transport of coliform bacteria in the ground water.