Two-week laboratory experiments were carried out in plastic boxes separated in two connected compartments filled with a neutral soil (mull humus) at pH 7.7 and an acid soil (moder humus) at pH 4.3, containing their original faunas. Migration of Collembola from one compartment to the other was allowed through a perforated wall. The mull was contaminated with three concentrations of lead (as lead acetate) at 50, 6000 and 60,000 ppm. When combined with moder in the adjacent compartment, six times more individuals and five times more species were observed in the mull at the highest concentration applied, compared to mull combined with itself. Parisotoma notabilis proved to be highly sensitive to lead, and shifted to the moder compartment even at the lowest concentration. Densities of other mull species such as Pseudosinella alba were affected by medium to high concentrations of lead but these species did not move to the moder soil despite their high motility. Acidophilic species living in moder only, such as Willemia anophthalma, Proisotoma minima and Xenylla tullbergi, colonized contaminated mull treatments with densities increasing with lead concentrations but the result of this process was erratic. Folsomia manolachei was present in both humus forms but was much more abundant in the moder. This species colonized mull at medium to high lead concentrations, where it restored totally or partly its original abundance in the uncontaminated mull. These results suggested differences between mull and moder populations of Folsomia manolachei.