Buoyant densities were determined for groundwater bacteria and microflagellates (protozoa) from a sandy aquifer (Cape Cod, MA) using two methods:  (1) density-gradient centrifugation (DGC) and (2) Stoke's law approximations using sedimentation rates observed during natural-gradient injection and recovery tests. The dwarf (average cell size, 0.3 μm), unattached bacteria inhabiting a pristine zone just beneath the water table and a majority (∼80%) of the morphologically diverse community of free-living bacteria inhabiting a 5-km-long plume of organically-contaminated groundwater had DGC-determined buoyant densities <1.019 g/cm³ before culturing. In the aquifer, sinking rates for the uncultured 2-μm size class of contaminant plume bacteria were comparable to that of the bromide tracer (1.9 × 10^-3 M), also suggesting a low buoyant density. Culturing groundwater bacteria resulted in larger (0.8−1.3 μm), less neutrally-buoyant (1.043−1.081 g/cm³) cells with potential sedimentation rates up to 64-fold higher than those predicted for the uncultured populations. Although sedimentation generally could be neglected in predicting subsurface transport for the community of free-living groundwater bacteria, it appeared to be important for the cultured isolates, at least until they readapt to aquifer conditions. Culturing-induced alterations in size of the contaminant-plume microflagellates (2−3 μm) were ameliorated by using a lower nutrient, acidic (pH 5) porous growth medium. Buoyant densities of the cultured microflagellates were low, i.e., 1.024−1.034 g/cm³ (using the DGC assay) and 1.017−1.039 g/cm³ (estimated from in-situ sedimentation rates), sug gesting good potential for subsurface transport under favorable conditions.