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This paper presents a novel design of a four-legged “spider” robot capable of moving in a wide range of two-dimensional tunnels. The robot moves in a quasistatic manner, by stably bracing itself against the tunnel walls while moving its free parts to the next position. The design has been strongly influenced by the recent immobilization theory of Rimon and Burdick (1998a, 1998b). The theory dictates the minimum number of limbs such a mechanism can have, as well as the curvature of the mechanism footpads. The class of tunnel geometries dictates other key parameters of the robot, such as limb dimensions and number of degrees of freedom of each limb. We review the relevant components of the immobilization theory and describe its implications for the robot design. Then we describe our choice of other key design parameters of the robot. The spider-like robot will move under a worst-case assumption of slippery tunnel walls, and we also describe a locomotion strategy for the robot under this assumption. Finally, we describe an immobilization-based control algorithm for executing the motion strategy. The robot has been built, and experiments verifying its robustness with respect to leg-placement errors are described.