A single 200‐μm particle is injected into a 0.25% w/w solution of a commercial polyisobutylene(Mv=1.5×106)in a polybutene solvent, held between a plate and a 21‐cm disc. When rotation of the disc at 5.7 rpm is begun with a disc‐plate separation of 5.4 mm, the particle initially located at a distancer5.1 cmmigrates away from the axis with about the same speed. This sharp reversal in the direction of migration with such a small change in the initial location is not expected on the basis of any current theory. Increasing the rate of rotation or decreasing the disc‐plate separation increases this criticalrfor reversal, while doubling the particle radius had no significant effect.