Erythrocytes from a chelonian (Geochelone carbonaria) were submitted to physical and chemical treatments. Observations of their change in shape from ellipsoidal to spherical were made by means of light microscopy. A 100% change to the spherical shape was attained with erythrocytes under high temperature treatment (42 degrees C) for 45 min., and at room temperature (29 degrees C) plus colchicine, for 60 min. Erythrocytes kept at room temperature present changes in shape while those kept at low temperature (10 degrees C) did not exhibit significant changes in their original ellipsoidal shape, but displayed an intact marginal band (MB). By direct examination the diameter of microtubules was found to be nearly 300 A in the dissociated form and 0.70 mu when bundled. In cross section the value found for microtubule diameter was up to 210 A. These measures are compatible with those found by several other authors. Counting of microtubules by ultrathin sectioning indicated approximately 70 tubules per cell, a value that is higher than that found in other species. By means of electron microscopy the marginal band can be observed intact or dissociated into several dense rings formed by parallel microtubules running nearly or accompanying the periphery of the stroma. Temperature and colchicine are responsible for the depolymerization of the microtubules. In the stroma of spherical erythrocytes a dense peripheric fold can be observed but microtubules were never found. It is suggested therefore that the role of the marginal band is essential to maintain the ellipsoidal shape of chelonian erythrocytes.