Virtual reality is often used to simulate environments in which the direction of up is not aligned with the normal direction of gravity or the body. How effective are these environments in terms of generating a compelling illusion of different up directions? Here we examine this question by asking: “In virtual reality, which way is up?”. Using an immersive projective display, subjects sat in a virtual room that could be rolled about the line of sight. Subjects indicted their perceived direction of up by adjusting the orientation of a shaded disk until it appeared maximally convex. This orientation depends upon the perceived direction of the illumination which thus indirectly indicates the perceived direction of up. Their judgements indicate that for physically upright subjects the visual display is an important factor in the perceived up direction. However this technique is limited to roll rotations away from the gravity direction in the range ±35°. Key words: human performance,...