Bacterioplankton was analyzed in monthly intervals from June to September in water samples from the meromictic Lake Cadagno (Switzerland) that was characterized by a permanent chemocline separating the aerobic epilimnion from the anaerobic, sulfidogenic hypolimnion. During the whole period, the physico-chemical parameters showed the characteristic stratification profile of Lake Cadagno though small spacial and temporal variations in the depth profiles of the different parameters were observed. Total bacterial numbers ranged from 0.5 (±0.4) 10⁶ to 11.0 (± 1.7) 10⁶ cells ml^-1 with maxima in the chemocline and in the underlying monimolimnion. At a depth of 13.4 m or at 13.5 m, morphotypes resembling Chromatium okenii or Amoebobacter purpureus occurred in highest numbers with up to 2.0 (±0.5) 10⁶ or 9.0 (±2.0) 10⁶ cells ml^-1, during July. In the monimolimnion, a rod shaped, vacuolated bacterium (morphotype R) reached its highest concentrurion of 4.4 (±0.6) 10⁶ cells ml^-1 at a depth of 19 m in June. In situ hybridization with fluorescent (Cy3-labeled) probe Eub338 designed to detect all organisms of the domain Bacteria resulted in the detection of up to 90% of the DAPI-stained cells in the chemocline and up to 70% in the anoxic monimolimnion. Averaged over the whole chemocline, 49 ±18% of the DAPl-stained bacteria hybridized to probe Gam42a targeting bacteria of the γ-subdivision of Proteobacteria, whereas cells hybridizing to probes Alf1b, Bet42a, and SRB385 targeting bacteria of the α-, β-, and δ-subdivisions of Proteobacteria, respectively, accounted for 23 ±3, 15 ±7, and 16 ±4%. In the monimolimnion, bacteria hybridizing to probe Alf1b were the most abundant organisms with 32 ±9% of the DAPl-stained bacteria. For the on average less abundant bacteria detected with probes Bet42a and SRB385, a higher diversity with respect to morphotypes was observed than for bacteria detecled with probes Alf1b and Gam42a throughout the whole water profile.