The acquisition of cellular asymmetry is one of the post-mitotic events in development. The cells of the avian erythropoietic lineage acquire a simple invariant asymmetry as they mature. Erythrocytes develop in suspension from spherical through discoid to lentil-shaped (lentiform) cells, with a single rigorously specified microtubule bundle, the marginal band. We show here that developing erythrocytes can express highly asymmetric morphologies, including elongated cell bodies and long processes in response to two stimuli: mechanical manipulation (repeated washing) and exposure to cytochalasin D. These experiments suggest that erythrocytes pass through a developmental stage during which microtubules are able to exert elongating forces on the cell. That stage is one in which these cells normally change shape, from spherical to discoid. The results suggest that microtubules may both guide and drive the formation of the marginal band and the characteristic morphology of these cells.