The soot distribution as function of ambient O₂ mole fraction in a heavy-duty diesel engine was investigated at low load (6 bar IMEP) with laser-induced incandescence (LII) and natural luminosity. A Multi-YAG laser system was utilized to create time-resolved LII using 8 laser pulses with a spacing of one CAD with detection on an 8-chip framing camera. It is well known that the engine-out smoke level increases with decreasing oxygen fraction up to a certain level where it starts to decrease again. For the studied case the peak occurred at an O₂ fraction of 11.4%. When the oxygen fraction was decreased successively from 21% to 9%, the initial soot formation moved downstream in the jet. At the lower oxygen fractions, below 12%, no soot was formed until after the wall interaction. At oxygen fractions below 11% the first evidence of soot is in the recirculation zone between two adjacent jets. The initial growth rate of the soot area, in the plane of the laser sheet, increases when the O₂ fraction decreases from 21% to 13%. Also the residence time in the soot area near CA50 increases during this part of the EGR-sweep. In combination, these trends are consistent with greater net soot production with decreasing O₂. It is reasonable to assume that the increased net soot production is partially associated with reduced soot oxidation at low O₂ fractions, although this cannot be quantified from the present data. Below 12% O₂, past the peak in smoke emissions, both the area growth rate and the residence time decrease. This should decrease the total net soot formation.