We found unidirectional nanocracks in unidirectionally twinned films of YBa2Cu3O7−δ on NdGaO3 which are normal to the twin planes. In 70–250 nm thick films, transmission electron and atomic force microscopy show that these unidirectional nanocracks are found to be 1–2 μm apart and have 2–4 nm width. Microbridges that cross these nanocracks behave as natural Josephson junctions. They show Shapiro steps under microwave irradiation and suppression of the critical current Ic in magnetic fields. A very large in-plane anisotropy of the resistivities and Ic was observed and this was correlated with the nanocracks orientation, length, and film thickness. The observed anisotropy of Ic near Tc is shown to be consistent with a d+is wave symmetry of the order parameter in YBa2Cu3O7−δ. © 1998 American Institute of Physics.