The tensile deformation behavior of single crystals of Ti3Al (Ti-25 at.% Al) deformed in tension parallel to the c-axis has been studied at room temperature as a function of the thickness of tensile specimens. While premature fracture always precedes any plastic flow when the specimen thickness is larger than 300 mu m, a 0.3-0.4% plastic strain is observed when the specimen thickness is reduced to 150 mu m. The slip plane observed in tension is {20 (2) over bar (1) over bar}, which is different from {11 (2) over bar (1) over bar} observed in compression. Assuming that slip on {20 (2) over bar (1) over bar} occurs along (11 (2) over bar 6), the critical resolved shear stress (CRSS) value is estimated to be 420 MPa, which is a little lower than 450 MPa obtained for slip on {11 (2) over bar (1) over bar}. The slip plane selection depending on the sense of applied stress (i.e. tension or compression) and the relative magnitude of CRSS values are consistent with the reported results of atomistic studies on the core structure of 1/3 [11 (2) over bar 3] dislocations in a model h.c.p. lattice. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.