The shape of aggregates of cells formed by positive dielectrophoresis (DEP) at interdigitated oppositely castellated electrodes under different conditions was investigated and compared with calculations of the electric field gradient vertical bar del E-2 vertical bar, and the electric field E, and E-2. The results confirm that at low field strength the cells predominantly accumulate above the tips of the electrodes, but at higher electric field strengths the cells predominantly accumulate in the middle of the aggregate. For a given electrode size, a higher applied voltage significantly increases the aggregate footprint. Higher flow rates distort this pattern, with more cells accumulating at the electrodes that are upstream. Calculation of the electric field strength E, E-2 and the electric field strength gradient vertical bar del E-2 vertical bar in the interdigitated oppositely castellated electrode array shows that, at low flow rates, there is a strong correlation between the aggregate shape and the distribution of the electric field E and E-2 but not so between the aggregate shape and vertical bar del E-2 vertical bar. The results indicate that interparticle forces such as pearlchain formation strongly affect the aggregation process, but that, when positive DEP is used to make the aggregates, the distribution of the electric field E, or better F-2 can be used as a useful guide to the final aggregate shape.