The aerobic, chemoheterotrophic bacteria indigenous to deep aquifers and other subsurface sediments (depths to 265 m) at a site in South Carolina were characterized by direct microscopy, enumeration of viable cells, analysis of colony morphologies on plates, and analysis of cell morphologies of isolated strains. Substantial numbers of viable bacteria (105‐108/g) were present in all transmissive, aquifer sediments, and their numbers did not decrease with depth. Fewer bacteria (<103/g) were detected in nontransmissive, confining layers. The highest viable counts were obtained on dilute media, but 10–50% of the bacteria in most aquifer sediments also grew rapidly on concentrated, nutrient‐rich media (indicating a high degree of metabolic flexibility). Most of the bacteria were mesophilic; relatively few psychrophiles or thermophiles were detected (<103/g; in many cases, none). The bacterial flora was diverse (11–62 distinct colony types on enumeration plates of most aquifer sediments). Diversity did not decrease with depth, but the composition of the microflora (based on colony analysis) varied extensively from one geological formation to another. Almost 95% of the platable colonies that grew on enumeration plates contained nonstreptomycete bacteria, more than 80% of which were gram‐negative rods. Light microscopy of films released from aquifer sediments by flotation revealed the presence of dividing cells and microcolonies, thus implying that the in situ deep aquifer microflora was more metabolically active than that seen previously in shallow aquifers.