Aerobic chemoheterotrophic bacteria were isolated from surface soils and coastal plain subsurface (including deep aquifer) sediments (depths to 265 m) at a study site near Aiken, S.C., by plating on concentrated and dilute media. Morphologically distinct colonies were purified, and their responses to 21 selected physiological tests were determined. These isolates were quite diverse; 626 physiologically distinct types (i.e., types with a unique pattern of responses to the 21 tests) were detected among the 1,112 isolates obtained. Physiologically distinct types were isolated on concentrated and dilute media (only 11% overlap between the groups); isolates from surface soils and subsurface sediments were also quite different (only 3% overlap). The surface soil isolates more readily utilized all but 1 of 12 carbon sources offered, and a significantly larger proportion of them hydrolyzed esculin and gelatin. Only 4% of the subsurface isolates fermented glucose, even though 82% of them could use it aerobically. l-Malate and d-gluconate were utilized by at least 75% of the subsurface isolates, and seven other carbon sources were used by at least 40% of them. Subsurface isolates from different geological formations (depths) and, to a lesser extent, from the same geological formation at different boreholes differed distinctly in their group responses to certain physiological tests. Moreover, sediments from different depths and boreholes contained physiologically distinct types of bacteria. Thus, considerable bacterial diversity was observed in coastal plain subsurface sediments, even within defined geological formations.