In major and intermediate thalassemia a decrease in erythrocyte deformability and increased erythrocyte aggregability has been described, but few studies have dealt with the question of rheological red blood cell behaviour in minor beta and deltabeta thalassemia carriers, mostly in deltabeta, because it is a less common entity. To ascertain whether there are differences in red blood cell behaviour between minor thalassemia and controls and between both types of thalassemia trait beta and deltabeta, we determined erythrocyte deformability and aggregability in 30 beta and 30 deltabeta trait carriers diagnosed both with conventional methods and globin gene analysis, and in 40 age- and sex-matched controls. Erythrocyte deformability determined by means of the Rheodyn SSD showed a statistically significant lower Elongation Index (EI) at all the shear stresses tested in both thalassemic groups compared with controls (p<0.001). Minor beta thalassemia carriers showed lower EI than deltabeta carriers (p<0.001). Erythrocyte aggregability measured with the Myrenne aggregometer was significantly lower in both thalassemic groups than in controls (p<0.001), although no significant differences could be observed between both thalassemic groups. The rheological alterations found in thalassemia carriers are in part due to microcytosis, hypochromia and the morphological changes that characterize this kind of anaemia. The less altered deformability found in deltabeta carriers, is in agreement with the fact that it deals with a more benign trait.