Since the discovery of graphene, layered materials have attracted extensive interest owing to their unique electronic and optical characteristics. Among them, Dirac semimetals, one of the most appealing categories, have been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe 5 was found to host signatures of a Dirac semimetal. However, the low Fermi level in ZrTe 5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe 5 through magneto-optics and magneto-transport. By applying a magnetic field, we observe a ffiffiffiffi B p dependence of the inter-Landau-level resonance and Shubnikov-de Haas (SdH) oscillations with a nontrivial Berry phase, both of which are hallmarks of Dirac fermions. The angle-dependent SdH oscillations show a clear quasi-two-dimensional feature with a highly anisotropic Fermi surface and band topology, in stark contrast to the three-dimensional (3D) Dirac semimetal such as Cd 3 As 2 . This is further confirmed by the angle-dependent Berry phase measurements and the observation of bulk quantum Hall effect (QHE) plateaus. The unique band dispersion is theoretically understood: the system is at the critical point between a 3D Dirac semimetal and a topological insulator phase. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe 5 as an ideal platform for exploring the exotic physical phenomena of Dirac fermions. NPG Asia Materials (2016) 8, e325; doi:10.1038/am.2016.166; published online 11 November 2016 INTRODUCTION Layered materials, formed by stacking strongly bonded layers with weak interlayer coupling, 1-10 have drawn immense attention in fundamental studies and device applications owing to their tunability in band structures and Fermi energy. 3,4,[11][12][13] Unlike other layered materials such as MoS 2 and BN, graphene stands out as an appealing candidate, as it is featured with a linear energy dispersion and low-energy relativistic quasi-particles. 9,14,15 Many exotic phenomena, such as a half-integer quantum Hall effect (QHE) 1,2 and Klein tunneling, 16 have been realized in graphene. Along this line, extensive efforts were also devoted to exploring new Dirac semimetal states in other layered systems beyond graphene. 5,6 Pentatelluride ZrTe 5 with a layered orthorhombic structure has been widely studied since the 1980s for its resistivity anomaly [17][18][19] and large thermopower. 20,21 For a long time, ZrTe 5 was considered to be a semimetal or degenerated semiconductor with a parabolic energy dispersion. 10,22 However, a recent study 7 revealed a linear dispersion in ZrTe 5 bulk states along with a chiral magnetic effect, hosting the signatures of a Dirac semimetal. Nevertheless, owing to the relatively