We report the first direct experimental evidence of intrinsic pinning in hysteresis loops of untwinned YBa2Cu307-"single crystals, for magnetic fields perpendicular to the c axis. We have observed oscillations in the M-0 loops that are well explained when one considers the layered structure of this compound. The oscillations are found to be periodic in 0, ' ' and are induced by a succession of transitions between states for which the vortex lattice is commensurate with the crystal lattice periodicity in the direction perpendicular to the Cu02 planes. The following anisotropy parameters were extracted from the experimental results: I,t, =(m, /mt, ) ' =6. 3, I "=(m,/m, ) ' =5. 6, and F,t, =(m, / mt, )' = l. l3. PACS numbers: 74.72.Bk, 74.25.Ha, 74.60.6e It is well established that the anisotropy in high-T, superconducting oxides plays an important role in their physical properties. However, many of these properties cannot be accounted for by a simple anisotropic 3D description because the layered structure of these materials confers some characteristic features on them. These characteristics are not specific to the cuprate superconductors; interest in this topic originated in the early seventies through the study of conventional layered materials [1]. The layered structure leads to interesting physical properties. For example, a 3D to 2D transition [2] is expected at a temperature T*, where the coherence length along the c axis becomes smaller than the spacing between the CuOz planes (the superconducting layers). Also, when a magnetic field H )H, ilsiapplied along the layers, it is energetically favorable at low temperatures for a vortex to lie between two Cu02 planes. Feinberg and Villard [2] have shown that for an applied magnetic field making an angle 0 with the a-b plane, the vortices lock in to the a-b plane if 8( 8, (the critical angle). Experimentally, an anomaly in torque measurements on an untwinned YBa2Cu307-"(YBCO) single crystal has been observed by Farrell et al. [3] and attributed to lock-in by Bulaevskii [4]. A similar anomaly was reported [5,6] for TlzBa2CaCuzOs and BizSrzCaCu20s single crystals. I n this paper, we present the first clear evidence of intrinsic pinning by the layered structure of high-T, superconductors observed in hysteresis loops of untwinned YBCO single crystals recorded for magnetic fields parallel to Cu02 planes. We wi11 also deduce accurately the anisotropy parameters I;~= (m;/ml) 't, where m; and mã re the effective masses defined such that the subscripts refer to the principal axes along which screening currents Aow.The crystals investigated were grown as described in Ref. [7] by a conventional self-flux method using yttria stabilized zirconia crucibles. They were oxygenated for 6 d at 500 C in an oxygen atmosphere. Detwinning was achieved by applying a uniaxial pressure of -50 MPa at 550 C in air for 15 min. Detwinned crystals were then reoxygenated for 1 d at 500'C in oxygen atmosphere, providing an overall oxygen content [8] 6.9 (7x & 6.92. Polarized optical microscopy...