It has been suggested that reentrant activity in three-dimensional cardiac muscle may be organized as a scroll wave rotating around a singularity line called the "lament. Experimental studies indicate that "laments are often concealed inside the ventricular wall and consequently, scroll waves do not manifest reentrant activity on the surface. Here we analyse how such concealed scroll waves are a!ected by a twisted anisotropy resulting from rotation of layers of muscle "bers inside the ventricular wall. We used a computer model of a ventricular slab (15;15;15 mm) with a "ber twist of 1203 from endocardium to epicardium. The action potential was simulated using FitzHugh}Nagumo equations. Scroll waves with rectilinear "laments were initiated at various depths of the slab and at di!erent angles with respect to "ber orientation. The analysis shows that independent of initial conditions, after a certain transitional period, the "lament aligns with the local "ber orientation. The alignment of the "lament is determined by the directional variations in cell coupling due to "ber rotation and by boundary conditions. Our "ndings provide a mechanistic explanation for the prevalence of intramural reentry over transmural reentry during polymorphic ventricular tachycardia and "brillation.