A novel input device called the Rockin'Mouse is described and evaluated. The Rockin'Mouse is a four degree-of-freedom input device that has the same shape as a regular mouse except that the bottom of the Rockin'Mouse is rounded so that it can be tilted. This tilting can be used to control two extra degrees of freedom, thus making it suitable for manipulation in 3D environments. Like the regular mouse, the Rockin'Mouse can sense planar position and perform all the usual functions. However, in a 3D scene a regular mouse can only operate on 2 dimensions at a time and therefore manipulation in 3D requires a way to switch between dimensions. With the Rockin'Mouse, however, all the dimensions can be simultaneously controlled. In this paper we describe our design rationale behind the Rockin'Mouse, and present an experiment which compares the Rockin'Mouse to the standard mouse in a typical 3D interaction task. Our results indicate that the Rockin'Mouse is 30% faster and is a promising device for both 2D and 3D interaction.