Surface-soil and subsurface microfloras at the site of a shallow aquifer in Oklahoma were examined and compared with respect to (1) total and viable cell numbers, (2) colony and cell types that grew on various plating media, (3) cell morphologies seen in flotation films stripped from sample particles, and (4) cellular ultrastructure. Appreciable numbers of microbial cells were present in the subsurface, but the subsurface microflora was considerably less populous than that of the surface soil. The subsurface microflora (especially that of the saturated zone) also appeared to be much less diverse, containing fewer microbial types that would grow on enumeration plates (on nutrient-rich media, 3-4 colony types versus 19-22 for the surface soil) and fewer cell types that could be distinguished by direct microscopy (3-4 types versus 17 for the surface soil). The specific types of microorganisms that were numerically predominant in the aquifer sediments were entirely different from those that were predominant in the surface soil. Moreover, the predominant types varied from one depth to another within the saturated zone. The potential metabolic capability of the subsurface microflora, as indicated by its readiness to grow rapidly on nutrient-rich media, also varied with depth. (Copyright (c) Springer-Verlag New York Inc. 1988.)