Theoretical expressions for electrorotation are derived for an ellipsoidal cell model with an arbitrary number of confocal shells. The theory for a spherical cell model with an arbitrary number of shells can be obtained as a special case of the present theory. Theoretical expressions are presented for several ellipsoidal cell models which are relevant to biological cells. The effects of cell shape and size on the electrorotation spectrum are discussed in detail for the case of the ellipsoidal single-shell model under the conditions that seem appropriate for biological cells.