The excitation mechanism of mercury atoms in a low‐pressure mercury‐argon discharge was investigated, employing an afterglow method. The experimental results combined with theoretical calculations have proved that the dominant process to populate the Hg 61P1level is the stepwise excitation via excited levels (63P−61P1) under conditions typical of fluorescent lamps (d=18.5–28.5 mm,I=0.3–0.5 A). When a Maxwell distribution of electron energy is assumed, the stepwise excitation is estimated to be about 73% of the total populating rate of the 61P1level in the stationary state. This value is about twice that found in literature. In the afterglow, the percentage goes beyond 97% within 10 &mgr;s after the termination of the current. By considering a non‐Maxwellian effect, the above partitions are estimated to decrease down to about 53% for the stationary state and to 93% for the afterglow. The stepwise excitation cross section was found to be in the order of 10−20–10−19m2.