Observations have been made of the low‐frequency fluctuations occurring in hot‐cathode dc mercury‐vapor discharges at a pressure of approximately 1 &mgr;. A spectrum analyzer was employed to study frequency spectra from three widely differing experimental tubes and to determine the effects of variations in tube geometry, electrode spacing, and anode current. Similar measurements were also made at points along a long discharge column with the aid of a capacitive probe sliding on the outside of the tube. The character of the spectrum is markedly dependent on tube geometry, substantially independent of column length and position along the column, and may be coarse or contain relatively pure frequency components. Two mechanisms observed involved frequency variation proportional to the square and cube roots of anode current, and possible explanations are given. There is, however, evidence for the existence of other basic mechanisms of generation.