Bacteriological investigations were carried out in the groundwater of sandy and gravelly deposits of the river Fulda valley in an area named "Johannesaue" near the town Fulda (Hesse, Fed. Rep. of Germany). In January 1979 water samples were collected from 16 pumping tubes distributed in organically polluted and unpolluted areas. For characterizing the bacterial populations, methods used for surface waters were modified and, as far as the author is aware, these methods were used for the first time for investigations pertaining subterranean waters. The bacteria were counted by means of epifluorescence microscopy after staining the bacteria with the fluorochrome acridine orange. This technique renders possible the simultaneous registration of shape and size of bacteria. Parameters characterizing the heterotrophic bacterial activity were measured with 14C-labelled glucose. The number of bacteria in the groundwater collected through pumping tubes, varied from 1.4 to 11.3 million bacteria per ml. The relative glucose uptake potential vr, which was measured at one substrate concentration (600 microg glucose .1-1) where the maximum uptake velocity is almost reached, was 0.12 - 0.74 microg glucose 1-1 .h-1. The corresponding specific potential was 0.02-0.18 microg glucose h-1 cell-1. The results agreed with the values of maximum uptake velocity Vmax which was measured at the same time in some of the groundwater samples. The data give first information about distribution of the number of bacteria and of heterotrophic bacterial activity in the groundwater of the investigation area. Relationships could be shown between the bacteriological parameters on the one hand and the concentration of oxygen and the values of COD measured with KMnO4 on the other hand.